softpipe: move all color-combine code into sp_quad_blend.c

Consolidate the read-modify-write color combining code from
the blend, colormask and output stages.

src/gallium/drivers/softpipe/SConscript

@@ -18,13 +18,11 @@ softpipe = env.ConvenienceLibrary(
 		'sp_quad_alpha_test.c',
 		'sp_quad_blend.c',
 		'sp_quad_pipe.c',
-		'sp_quad_colormask.c',
 		'sp_quad_coverage.c',
 		'sp_quad_depth_test.c',
 		'sp_quad_earlyz.c',
 		'sp_quad_fs.c',
 		'sp_quad_occlusion.c',
-		'sp_quad_output.c',
 		'sp_quad_stencil.c',
 		'sp_quad_stipple.c',
 		'sp_query.c',