added stub SSE backend
use -ffast-math on gcc
stub opengl backend
there is now one definitive list of backends, from which others are generated
allowed conditional compilation of OPENGL and CPU_SSE backends
added tests for opengl backend
added opengl triangle drawer
first version of composite for opengl
fixes to bring opengl backend in-line with cpu backend
removed redundant gl bind calls, and glenables
removed attempt at squashing repeated load_matrix calls
vertices_length, and shaders for the opengl backend
added back modelview matrix caching
added oil_image_clear
-O2 and unexported python type objects