* app/composite/gimp-composite.[ch],
* app/composite/make-gimp-composite-dispatch.py
stubbed out the 32bit-per-channel pixel formats.

* app/composite/gimp-composite-sse.[ch],
* app/composite/Makefile.am
Broke-out and added the SSE versions of the MMX code.

* app/composite/gimp-composite.[ch],
* app/paint-funcs/paint-funcs.c,
* app/composite/make-gimp-composite-dispatch.py
Use of the new gimp_composite code is now turned on via the
environment variable GIMP_COMPOSITE with a non-zero value.  For
example:
% GIMP_COMPOSITE=1 gimp-1.3