A few more bitmap icon updates, edge cases.

* Support downscaling icons to a size smaller than the source.
* For > 4x icon scaling do a scale4x followed by a bilinear scale.

Note that I tried to do a combination of scale2x/scale3x with bilinear scaling
and the results were worse than scale2x/scale3x with down scaling.

The 24x24 icon case looks pretty bad either using bilinear or scale2x followed
by a downscale because I am currently upscaling the 16x16 icon in Deskbar (we
didn't up until now support bitmap icon downscaling so I had no choice). It
might be a better idea to downscale the 32x32 version instead.

Note that all of the above has to do with bitmap icons ONLY and none of it
applies to HVIF icons that scale beautifully without these tricks.