Update bitmap downscaling for BeOS icons.

Implemented a simple down sampling algorithm in the scale_down() function. For
non-integer scaling first scale up using the scale2x, scale3x, or scale4x
algorithm doubling, tripling, or quadrupling the icon then use the downscaling
algorithm to shrink to the desired size. This produces nicer looking results
than bilinear scaling alone.

Note that this only applies to bitmap-based BeOS icons and not vector-based
HVIF icons.