GL_NV_framebuffer_multisample_coverage
http://developer.download.nvidia.com/opengl/specs/GL_NV_framebuffer_multisample_coverage.txt
	GL_RENDERBUFFER_COVERAGE_SAMPLES_NV 0x8CAB
	GL_RENDERBUFFER_COLOR_SAMPLES_NV  0x8E10
	GL_MAX_MULTISAMPLE_COVERAGE_MODES_NV 0x8E11
	GL_MULTISAMPLE_COVERAGE_MODES_NV 0x8E12
	void glRenderbufferStorageMultisampleCoverageNV (GLenum target, GLsizei coverageSamples, GLsizei colorSamples, GLenum internalformat, GLsizei width, GLsizei height)