Wayland: Fix alpha blending

- each window in wayland has its own buffer/texture and these are always
composited with alpha blending
- as a result any alpha blended areas of our UI will allow the
underlying window to be visible if the window/surface buffer has a
buffer with alpha
- usually the default surface format does not request a buffer with
alpha but when wayland decorations are enabled, Qt overrides the alpha
depth
- so as a workaround, disable Qt wayland decorations, which we don't
need anyway
- note - this may not be the best solution. Using
wl_surface_set_opaque_region on our surface would allow the compositor
to optimise rendering as it knows it does not need to show anything
hidden by the window. In testing this works but requires linking to
libwayland-client and including Qt private headers (which is far from
ideal)

- refs #13483

mythtv/libs/libmythui/mythdisplay.cpp

@@ -1051,6 +1051,7 @@ void MythDisplay::ConfigureQtGUI(int SwapInterval, const QString& _Display)
 {
     // Set the default surface format. Explicitly required on some platforms.
     QSurfaceFormat format;
+    format.setAlphaBufferSize(0);
     format.setDepthBufferSize(0);
     format.setStencilBufferSize(0);
     format.setSwapBehavior(QSurfaceFormat::DoubleBuffer);
@@ -1063,6 +1064,20 @@ void MythDisplay::ConfigureQtGUI(int SwapInterval, const QString& _Display)
     // of the MythPushButton widgets, and they don't use the themed background.
     QApplication::setDesktopSettingsAware(false);
 #endif
+
+    // If Wayland decorations are enabled, the default framebuffer format is forced
+    // to use alpha. This framebuffer is rendered with alpha blending by the wayland
+    // compositor - so any translucent areas of our UI will allow the underlying
+    // window to bleed through.
+    // N.B. this is probably not the most performant solution as compositors MAY
+    // still render hidden windows. A better solution is probably to call
+    // wl_surface_set_opaque_region on the wayland surface. This is confirmed to work
+    // and should allow the compositor to optimise rendering for opaque areas. It does
+    // however require linking to libwayland-client AND including private Qt headers
+    // to retrieve the surface and compositor structures (the latter being a significant issue).
+    // see also setAlphaBufferSize above
+    setenv("QT_WAYLAND_DISABLE_WINDOWDECORATION", "1", 0);
+
 #if defined (Q_OS_LINUX) && defined (USING_EGL) && defined (USING_X11)
     // We want to use EGL for VAAPI/MMAL/DRMPRIME rendering to ensure we
     // can use zero copy video buffers for the best performance (N.B. not tested