This would allow to immediately detect unintended writes out of
bounds like the ones fixed in
72b53807 and
1c7433a5.

Extend the PIXEL_RECT macro to provide a variable containing the
full, padded height of the buffer, for uses that operate on the
full buffer.

Allow overwriting past the right edge of the target output rectangle,
up to an alignment of 64 pixels, but allow no overwrite past the
bottom.