CELL: fleshing out the blending fragment ops

CELL: fleshing out the blending fragment ops

- Added two new debug flags (to be used with the CELL_DEBUG environment
  variable).  The first, "CELL_DEBUG=fragops", activates SPE fragment
  ops debug messages.  The second, "CELL_DEBUG=fragopfallback", will
  eventually be used to disable the use of generated SPE code for
  fragment ops in favor of the default fallback reference routine.
  (During development, though, the parity of this flag is reversed:
  all users will get the reference code *unless* CELL_DEBUG=fragopfallback
  is set.  This will prevent hiccups in code generation from affecting
  the other developers.)

- Formalized debug message usage and macros in spu/spu_main.c.

- Added lots of new code to ppu/cell_gen_fragment.c to extend the
  number of supported source RGB factors from 4 to 15, and to
  complete the list of supported blend equations.

More coming, to complete the source and destination RGB and alpha
factors, and to complete the rest of the fragment operations...