Before this change, using gl_MeshPrimitivesEXT in mesh shader would
unconditionally create gl_MeshPrimitivesEXT.gl_PrimitiveShadingRateEXT
field and add PrimitiveShadingRateKHR capability to the output SPIRV
file, which would subsequently trigger validation errors when creating
the shader module unless the application requested primitive shading
rate feature.
What should happen instead is that unless GL_EXT_fragment_shading_rate
extension is enabled, we should not allow using
gl_PrimitiveShadingRateEXT and should not emit the associated fields
into the output.
This change fixes this by using existing filterMember mechanism that is
already used in a few other cases like this, and adjusting the required
extension on the field member which will generate an error when
gl_PrimitiveShadingRateEXT is used without enabling the extension.
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