Add implementation of SPV_EXT_fragment_fully_covered

This implementation uses the GLSL extension
GL_NV_conservative_raster_underestimation to generate the new SPIR-V
FullyCoveredEXT built in.

Test/spv.fullyCovered.frag

@@ -0,0 +1,9 @@
+#version 450
+
+#extension GL_NV_conservative_raster_underestimation : enable
+
+out vec4 color;
+
+void main() {
+    color = vec4(gl_FragFullyCoveredNV, 0, 0, 0);
+}