* app/composite/gimp-composite-generic.c
* app/composite/gimp-composite-mmx.c
* app/composite/gimp-composite-sse.c
Incorporated a very clean patch from starox-gimp@starox.org
(Frederic Leroy) which improves the generic performance of the
burn compositing function.  Speed is improved at a cost of a 64k
look-up table which is probably manageable for 8bpp images, but at
larger bpp images (which currently are not supported by the GIMP)
this is unlikely to be tolerable.

The generic C implementation of the burn function uses this
look-up table, the mmx/sse implementations have been commented out.