Move all code to do debugging spew into pixman-private.

Move all code to do debugging spew into pixman-private.

Rather than the region code having its own little debug system, move
all of it into pixman-private where there is already return_if_fail()
macros etc. These macros are now enabled in development snapshots and
nowhere else. Previously they were never enabled unless you modified
the code.

At the same time, remove all the asserts from the region code since we
can never turn them on anyway, and replace them with
critical_if_fail() macros that will print spew to standard error when
DEBUG is defined.

Finally, also change the debugging spew in pixman-bits-image.c to use
return_val_if_fail() instead of its own fprintf().