background: Limit mipmap levels to avoid loss of visible detail
When the wallpaper image is larger than the monitor resolution we already
use mipmapping to scale it down smoothly in hardware. We use
GL_LINEAR_MIPMAP_LINEAR for the highest quality
scaling that GL can do. However that option is designed for 3D use cases
where the mipmap level is changing over time or space.
Since our wallpaper is not changing distance from us we can improve the
rendering quality even more than
GL_LINEAR_MIPMAP_LINEAR. To do this we
GL_TEXTURE_MAX_LEVEL (if available) to limit the mipmap level or
blurriness level to the lowest resolution (highest level) that is still
equal to or higher than the monitor itself. This way we get the benefits
of mipmapping (downscaling in hardware) and retain the maximum possible
sharpness for the monitor resolution -- something that
GL_LINEAR_MIPMAP_LINEAR alone doesn't do.
Monitor is 1920x1080
Wallpaper photo is 4000x3000
Mipmaps stored on the GPU are 4000x3000, 2000x1500, 1000x750, ...
Before: You would see the average of the 2000x1500 and 1000x750 images.
After: You will now only see the 2000x1500 image smoothly scaled down.
Prerequisite: !1004 (merged)
Trivia: This seems to be the same approach as YouTube uses for its video thumbnails.