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The AppImage launcher exported the superseded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.

src/slic3r/GUI/Widgets/WebView.cpp

@@ -417,6 +417,17 @@ bool WebView::NeedsCompositingWorkaround()
 #endif
 }
 
+bool WebView::SupportsForceSharedMemory()
+{
+#ifdef __linux__
+    const guint major = webkit_get_major_version();
+    const guint minor = webkit_get_minor_version();
+    return major > 2 || (major == 2 && minor >= 44);
+#else
+    return false;
+#endif
+}
+
 #if wxUSE_WEBVIEW_EDGE
 bool WebView::CheckWebViewRuntime()
 {