Linux: only disable WebKit compositing for WebKitGTK older than 2.42 (#16385)

* Linux: only disable WebKit compositing for WebKitGTK older than 2.42

The default Wayland path set WEBKIT_DISABLE_COMPOSITING_MODE=1 whenever the
session had both DISPLAY and WAYLAND_DISPLAY, i.e. any Wayland session with
XWayland. That forces WebKitGTK accelerated compositing off and also disables
threaded scrolling, so the Device tab web view (Mainsail/Fluidd) lagged unless
the app was launched with GDK_BACKEND=x11, the only path that skipped the
variable. Disabling compositing also turns off threaded scrolling.

The XWayland compositing bug the fallback worked around is fixed in WebKitGTK
>= 2.42, so apply it only for older WebKit. WebView::NeedsCompositingWorkaround()
reads the runtime WebKitGTK version; the X11 opt-in path is unchanged.

Behavior is unchanged on WebKit < 2.42, on pure X11, and on native Wayland
without XWayland. The wxWidgets skill reference is updated to match.

* Linux: use WebKit's shared-memory transport for the NVIDIA DMABUF renderer

The Device tab web view is slow on NVIDIA proprietary drivers even with
accelerated compositing left on: since the DMABUF renderer became the
default in WebKitGTK 2.42, the driver cannot back it, and web views fall
back to a slow path. Gating the old WEBKIT_DISABLE_COMPOSITING_MODE
fallback on WebKit < 2.42 is the wrong axis, because the failure is
version-independent from 2.42 on.

Force the shared-memory transport with WEBKIT_DMABUF_RENDERER_FORCE_SHM=1.
That keeps compositing and threaded scrolling and skips only the hardware
path. Also set WEBKIT_FORCE_DMABUF_RENDERER=1, which clears the isNVIDIA()
early return that Debian/Ubuntu/openSUSE patch into
rendererBufferTransportMode() and which otherwise returns before the
FORCE_SHM check is reached.

Both are set before the X11-vs-Wayland split so the workaround also covers
XWayland, and only when WebKitGTK is new enough to honour FORCE_SHM
(>= 2.44, WebView::SupportsForceSharedMemory()).

The AppImage launcher exported the superseded
WEBKIT_DISABLE_DMABUF_RENDERER=1, which is read first, leaves the transport
set empty and so disables compositing outright; replace it with the same
pair. The wxWidgets skill reference is updated to match.

* Linux: drop the AppImage WebKit DMABUF exports; correct the skill notes

The launcher cannot gate on the runtime WebKitGTK version, so exporting
WEBKIT_DMABUF_RENDERER_FORCE_SHM/WEBKIT_FORCE_DMABUF_RENDERER there would
clear a distro isNVIDIA() guard on hosts older than 2.44, where FORCE_SHM
is ignored, re-exposing the broken hardware path. The binary already sets
both with the correct gate, so remove them and say who owns the workaround.

Also correct two claims in the skill reference. An empty transport set only
means "no accelerated compositing" from 2.44 on; 2.42/2.43 still fall back
to the Wayland/X11 renderers, removed in the same release that added
FORCE_SHM. And threaded scrolling is off by default in 2.44/2.45 (on from
2.46), so FORCE_SHM keeps compositing rather than scrolling. Document the
module-level detection cost and the WEBKIT_DMABUF_RENDERER_FORCE_SHM=0
opt-out.

* Flatpak: force WebKit's shared-memory transport instead of disabling DMABUF

The entrypoint exported WEBKIT_DISABLE_DMABUF_RENDERER=1 for NVIDIA. WebKit
reads that variable first and leaves the transport set empty, which turns
accelerated compositing off, so the Device tab still lagged for NVIDIA
Flatpak users, and the binary could not override it because its setenv is
non-replacing.

The runtime's WebKitGTK is vanilla, so there is no distro isNVIDIA() guard
to clear and WEBKIT_DMABUF_RENDERER_FORCE_SHM=1 alone is the right switch:
compositing stays on and only the hardware DMABUF path is skipped.

* Flatpak: drop the WebKit NVIDIA export; the binary owns the workaround

/proc/driver/nvidia/version is readable inside the Flatpak sandbox, so the
binary's NVIDIA branch already sets WEBKIT_DMABUF_RENDERER_FORCE_SHM with
the correct runtime gate and the entrypoint's own export is redundant.

Verified with:
  flatpak run --command=cat com.orcaslicer.OrcaSlicer /proc/driver/nvidia/version
which prints the NVRM version (driver 615.71.09) from inside the sandbox.

* Linux: say what the XWayland compositing gate actually tests

DISPLAY and WAYLAND_DISPLAY are both set in a native Wayland session as
well, because GNOME and KDE run Xwayland as part of the session and export
DISPLAY for it whether or not this process is an X client. The pair means
XWayland is available in the session, not that GTK picked X11 for this
process, so "(an XWayland session)" reads as more than the condition
establishes.

Say what the pair means, and name the case the fallback actually covers:
the X11 backend inside a Wayland session, which is what the forced
GDK_BACKEND=x11 above produces in a build without EGL.

Comment only; behaviour is unchanged.

* Linux: apply the XWayland compositing fallback only where GTK picks X11

The default path set WEBKIT_DISABLE_COMPOSITING_MODE whenever DISPLAY and
WAYLAND_DISPLAY were both set, but a native Wayland app inherits both, so a
build with EGL disabled accelerated compositing — and with it threaded
scrolling — while running on Wayland, where the XWayland bug does not apply.

The only default-path case that really runs through XWayland is the forced
GDK_BACKEND=x11 used when wx lacks EGL, so set the variable there. The
backend cannot be re-checked later: that branch exports GDK_BACKEND itself,
so the force and a user's GDK_BACKEND=x11 opt-in, which deliberately keeps
compositing, would be indistinguishable.

Behaviour is unchanged for WebKitGTK >= 2.42, where the gate already skipped
the variable, and for builds without EGL. A build with EGL on WebKitGTK <
2.42 in a Wayland session no longer loses compositing and threaded scrolling.

The wxWidgets skill reference is updated to match.

---------

Co-authored-by: SoftFever <103989404+SoftFever@users.noreply.github.com>

scripts/flatpak/entrypoint

@@ -1,7 +1,7 @@
 #!/usr/bin/env sh
 
-# Work-around https://gitlab.gnome.org/GNOME/gnome-build-meta/-/issues/754
-grep -q org.freedesktop.Platform.GL.nvidia /.flatpak-info && export WEBKIT_DISABLE_DMABUF_RENDERER=1
+# WebKit's NVIDIA DMABUF workaround (gnome-build-meta#754) is applied by the
+# binary itself, gated on the runtime WebKitGTK version (src/OrcaSlicer.cpp).
 
 # Work-around https://github.com/bambulab/BambuStudio/issues/3440
 # Use LC_NUMERIC instead of LC_ALL to prevent decimal separator issues