The GL_OES_blend_func_separate extension extends the blending capability
by defining a function that allows independent setting of the RGB and
alpha blend factors for blend operations that require source and
destination blend factors. It is not always desired that the blending
used for RGB is also applied to alpha.

For more information, see the OpenGL ES 1.1 Extension Pack document in
the Khronos API Registry:

    http://www.khronos.org/registry/gles/