When loading preview images from disk, we'd first scale them to the
standard preview image size (in `icon_copy_rect()`) and then scale them
again to the drawing size when eventually drawing to screen. The first
scaling would happen on the CPU, which is slow, and without filtering.
Now the image is stored in its original size and only scaled when
drawing, which uses scaling on the GPU with mipmaps and bi-linear
filtering. While a bit more blurry, the resulting image has less
artifacts and represents the original image better. Keeping the images
unscaled means memory footprint is bigger, we could cap the size if
necessary.
Noticed while working on #131871. Asset shelf previews would have more
artifacts than before.
See pull request for comparisons.
Pull Request: https://projects.blender.org/blender/blender/pulls/133559
315e7e04a8