GL_EXT_blend_func_separate
http://oss.sgi.com/projects/ogl-sample/registry/EXT/blend_func_separate.txt
	GL_BLEND_DST_RGB_EXT 0x80C8
	GL_BLEND_SRC_RGB_EXT 0x80C9
	GL_BLEND_DST_ALPHA_EXT 0x80CA
	GL_BLEND_SRC_ALPHA_EXT 0x80CB
	void glBlendFuncSeparateEXT (GLenum sfactorRGB, GLenum dfactorRGB, GLenum sfactorAlpha, GLenum dfactorAlpha)