Fix user breakpoints and 16/16z pixel format bilinear dma

[cubanismo]

See the individual commit messages for details. This pair of changes fixes two issues I ran into debugging my app tonight:

• Bilinear DMA reads of 16 bit color + 16 bit Z double and triple buffer pixel formats weren't supported even though writes were.
• The user breakpoint mechanism wasn't terribly usable in practice.

[toxieainc]

Nice!!