Track 'tainted' area of Bitmaps to optimize blit operations

The 'tainted' area of a Bitmap describes what parts are no
longer in a 'cleared' state. When we blit to a fully cleared
are of a Bitmap at full opacity, we can completely disregard
the existing pixels in the operation, meaning we can skip any
blending calculations and just blit / upload straight to the
texture. This greatly speeds up text message rendering.

In the process, pixman has become a new dependency for mkxp,
but the results of this optimization are well worth it!
Return advance as width for italic characters