WIP: parallax scrolling wallpaper

[PandorasFox]

needs integration with: lockscreen, wallpaper transition shaders, and a performance/optimization pass

a bit choppier than my lazy WpViewport solution, but I'm using a 75MiB image that's approx 7500x11000 with my 3440x1440 display - right now I'm testing with QT_IMAGEIO_MAXALLOC=0 qs -p . just to be able to load my gigantic wallpaper, so the perf issues I see might be solved by optimizing the image itself a bit, but I'm still doing a pass over the anchoring/positioning, rects & clipping, etc to tighten that down a bit first.

vibe-coded most of the first-pass impl here; rest of the work will be manual bits

current thoughts:

• when finishing a scroll, we could try and copy over the display-size crop over to the active wallpaper image slot for the shaders to use
• similar stuff for loading/cropping NextWallpaper
• (wallpaper picker widget would benefit from enabling mipmaps on the thumbnailed wallpapers since they're downscaled?)
• should just be able to export the scrolling percentage (or raw x/y offsets) to the lockscreen/blurred wallpaper widgets?