When downscaling raster brushes, we currently resample the original
brush with no fitlering (other than bilinear interpolation), which
results in very noticable aliasing when using heavily downscaled
big brushes.

This commit constructs a box-filtered mipmap hierarchy for the
original brush on-demand, and the closest mipmap is used as the
resampling source for downscaled brushes, significantly improving
the output quality.