cover-loader: Use the logical size for all but rendering

This doesn't preemplively apply the scale factor to the rendering size
to keep it be the logical size. Only apply the scale when rendering.

This also makes the 256@2x cover cache hold a 512x512 image for the
256x256 logical size, rather than a 256x256 image for the 128x128
logical size, making it more intuitive.
4 jobs for cover-cache-path in 132 minutes and 46 seconds (queued for 15 minutes and 54 seconds)