GL_EXT_blend_color
http://oss.sgi.com/projects/ogl-sample/registry/EXT/blend_color.txt
	GL_CONSTANT_COLOR_EXT 0x8001
	GL_ONE_MINUS_CONSTANT_COLOR_EXT 0x8002
	GL_CONSTANT_ALPHA_EXT 0x8003
	GL_ONE_MINUS_CONSTANT_ALPHA_EXT 0x8004
	GL_BLEND_COLOR_EXT 0x8005
	void glBlendColorEXT (GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha)