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.

src/slic3r/GUI/Widgets/WebView.cpp

@@ -69,6 +69,10 @@ webkit_web_view_run_javascript_finish                (WebKitWebView
 						      GError                    **error);
 WEBKIT_API void
 webkit_javascript_result_unref              (WebKitJavascriptResult *js_result);
+WEBKIT_API guint
+webkit_get_major_version                     (void);
+WEBKIT_API guint
+webkit_get_minor_version                     (void);
 }
 #endif
 
@@ -401,6 +405,18 @@ bool WebView::NeedsRecreateOnShow()
     return false;
 #endif
 }
+
+bool WebView::NeedsCompositingWorkaround()
+{
+#ifdef __linux__
+    const guint major = webkit_get_major_version();
+    const guint minor = webkit_get_minor_version();
+    return major < 2 || (major == 2 && minor < 42);
+#else
+    return false;
+#endif
+}
+
 #if wxUSE_WEBVIEW_EDGE
 bool WebView::CheckWebViewRuntime()
 {