cogl_blend_func caches blending setup much like cogl_enable
does with the enable flags. This separates blending factors
setup from the enable/disable operation in preparation of
the texture image retrieval fix for alpha channel on GLES.
(cogl_enable:) Does not modify blending factors anymore.
* clutter/cogl/gl(es)/cogl-context.h: CoglContext holds two
new variables to cache blending src and dst factors.
* clutter/cogl/gl(es)/cogl-context.c:
(cogl_create_context:) Initialize blending factors.
* clutter/cogl/gles/cogl-texture.c:
(cogl_texture_download_from_gl:) Set blending factors to
CGL_ONE, CGL_ZERO which fixes the slighlty improper behavior
where source colour was actually multiplied with its alpha
value in the result (not noticable on current tests).
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