Implement GL_EXT_fragment_invocation_density

including SPV generation using SPV_EXT_fragment_invocation_density.
This is an alias of the functionality in SPV_NV_shading_rate, and thus in some
cases we can only have one set of the tokens present (switch statements), so
we have picked the EXT version. This required updating the expected test
results for SPV_NV_shading_rate.

Also updated the known-good for spirv-headers so that the validator in
spirv-tools knows about the new extension.

Test/spv.fragmentDensity.frag

@@ -0,0 +1,11 @@
+#version 450
+
+#extension GL_EXT_fragment_invocation_density : require
+
+layout (location = 0) out vec2 FragSize;
+layout (location = 2) out int FragInvocationCount;
+
+void main () {
+    FragSize = gl_FragSizeEXT;
+    FragInvocationCount = gl_FragInvocationCountEXT;
+}