Implement SPV_KHR_post_depth_coverage

Added support for both extension GL_ARB_post_depth_coverage and GL_EXT_post_depth_coverage.

Test/spv.extPostDepthCoverage.frag

@@ -0,0 +1,9 @@
+#version 310 es
+
+#extension GL_EXT_post_depth_coverage : enable
+
+layout(post_depth_coverage) in;
+layout(early_fragment_tests) in;
+
+void main () {
+}