Add support for SPV_NV_sample_mask_override_coverage

Test/spv.sampleMaskOverrideCoverage.frag

@@ -0,0 +1,7 @@
+#version 450
+#extension GL_NV_sample_mask_override_coverage : enable
+in vec4 color;
+layout(override_coverage) out int gl_SampleMask[];
+void main() {
+    gl_SampleMask[0] = int(0xFFFFFFFF);
+}