* composite/ modularisation of the automatic compositing function
  "wiring" to construct the table of compositing functions at
  run-time instead of compile time.

  The environment variable GIMP_COMPOSITE is a hexadecimal number
  representing a bit-field used to set/reset options in the
  gimp-composite code.

* gimp-composite-altivec.[ch]: added
* gimp-composite-altivec-installer.[ch]: added
* gimp-composite-vis.[ch]: added
* gimp-composite-vis-installer.[ch]: added
* gimp-composite-mmx-installer.[ch]: added
* gimp-composite-sse-installer.[ch]: added
* gimp-composite-altivec-test.[ch]: added
* gimp-composite-vis-test.[ch]: added
* gimp-composite-mmx-test.[ch]: added
* gimp-composite-sse-test.[ch]: added
* make-installer.py: Added meta programme for generating code to
  1) construct a table of compositing functions indexed by
  operation, and the pixel formats of the sources and destination,
  and 2) generating code for regression testing of these compositing
  functions against the same functions in gimp-composite-generic.c

* gimp-composite-mmx.c (xxxgimp_composite_overlay_rgba8_rgba8_rgba8_mmx):
  removing from the current set of optimisations because it appears
  to be broken.

* Makefile.am: modified regression testing targets to now build
  gimp-composite-*-test where * is any of generic, mmx, sse,
  altivec, and vis.

* gimp-composite.c: moved in initialisation code
  gimp_composite_init() from gimp-composite-dispatch.c

* gimp-composite.c: removed gimp_composite_unsupported().

* gimp-composite.c: use cpu_accel() to determine at run-time which
  set of optimisations are to be used.

* gimp-composite.c: conditionally compile code for ARCH_X86.

* make-gimp-composite-dispatch.c: retired.  See make-installer.py

* gimp-composite-mmx.c (op_overlay): namespace cleanup, op_overlay()
  and all of the constant values are now static

* gimp-composite-mmx.c, gimp-composite-sse.c: fixed bug with the
  alpha mask being the address of the alpha mask, not the mask
  itself.

* app/paint-funcs/paint-funcs.c: Check gimp_composite_options.use
  to see if we should use the new code or the old code.