Fix for Designer mistakenly using ARGB visuals

This is a workaround for Designer where it mistakenly uses the ARGB visual when the X11 Composite extension is enabled, causing Designer to be washed out. This forces Xlib to hide the ARGB visual just for Designer and not other Qt2 apps that work correctly. Qt2 appears to be able to correctly obtain a TrueColor visual, but for some reason Designer still grabs the ARGB visual, and then looks terrible. This fixes this on X11 and XWayland, and will not affect other backends.