texture: Enable texture diff against arbitrarily distant ancestors

Replaces single hop gdk_texture_diff with an implementation that will traverse the full history of (still referenced) previous textures.

For background, I have been working on an RDP client that renders a single animated DMA buffer across multiple fullscreen windows. Damage tracking is done by creating a new GdkTexture for each frame. There are a few situations where GTK will miss a frame (monitors having different refresh rates being the most reproducible), causing the entire screen to be invalidated and often resulting in a cascade of frame misses and full invalidations until the server detects the drop in frame rate and slows things down. This patch allows missed frames to be recovered from gracefully.