The generic implementations of many of the Blit (and BlitBg and MaskBlit) loops
do not adequately check the clip region before they determine the amount of
intermediate work to do and how big they should allocate any intermediate
buffers.

They all honor the clip eventually when it comes time to write their output
to the destination drawing surface, but they could perform a lot of intermediate
work that will never appear on the screen in the meantime.