GL_OES_blend_subtract
https://www.khronos.org/registry/OpenGL/extensions/OES/OES_blend_subtract.txt
GL_OES_blend_subtract

	GL_FUNC_ADD_OES 0x8006
	GL_BLEND_EQUATION_OES 0x8009
	GL_FUNC_SUBTRACT_OES 0x800A
	GL_FUNC_REVERSE_SUBTRACT_OES 0x800B
	void glBlendEquationOES (GLenum mode)