Fix background texture corruption when resuming from suspend on Nvidia
Mutter already has the
gl-video-memory-purged signal to support this feature of the Nvidia driver (https://www.khronos.org/registry/OpenGL/extensions/NV/NV_robustness_video_memory_purge.txt).
We just needed that signal hooked up to more parts of the desktop.
Note: The only goal of this branch is to fix the background wallpaper (https://bugs.launchpad.net/bugs/1809407) for Xorg sessions. Any other related fixes that don't easily fit into this branch should be proposed and discussed separately. That includes the native backend.