* gimp-composite-sse2-installer.c, gimp-composite-sse2-test.c:
  regenerated with new functions.

* gimp-composite-sse.c, gimp-composite-sse2.c:
  Distinguish between 64bit and 128bit constants with a little
  faux hungarian notation.

* gimp-composite-sse2.[ch]: Added implementations of
  addition_rgba8_rgba8_rgba8, subtract_rgba8_rgba8_rgba8, and
  swap_rgba8_rgba8_rgba8

* gimp-composite-generic.c:
  Some formating beautification