Bug 1699864 - Request 32bit color depth on Linux/EGL by default, r=stransky,jgilbert

Since D108508 the X11/EGL backend creates a shared GL context via
`CreateGLContextEGL()` which chains up to `CreateForCompositorWidget()`
with a `nullptr` widget. With the OGL compositor we relied on the
widget giving us a valid color depth, while now we'd fall back to
`gfxVars::ScreenDepth()`.

On X11 color depth is defined as:
> depth means the number of bits in a pixel that are actually used
> to determine the pixel color

i.e. we on a usual system we would get 24bit.

As we require an alpha channel when using WR, the result would be
disappointing. Thus hardcode 32bit color depth for X11/EGL when
creating contexts without widget for now.

In order to keep X11 and Wayland code close together, do the same
on Wayland, even if `gfxVars::ScreenDepth()` returns valid values
there.

Differential Revision: https://phabricator.services.mozilla.com/D109737

gfx/gl/GLContextProviderEGL.cpp

@@ -1007,6 +1007,10 @@ already_AddRefed<GLContext> GLContextProviderEGL::CreateForCompositorWidget(
     window = GET_NATIVE_WINDOW_FROM_COMPOSITOR_WIDGET(aCompositorWidget);
 #if defined(MOZ_WIDGET_GTK)
     depth = aCompositorWidget->AsX11()->GetDepth();
+#endif
+  } else {
+#if defined(MOZ_WIDGET_GTK)
+    depth = 32;
 #endif
   }
   return GLContextEGLFactory::Create(window, aWebRender, depth);