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.

SPIRV/doc.cpp

@@ -49,6 +49,7 @@ namespace spv {
     extern "C" {
         // Include C-based headers that don't have a namespace
         #include "GLSL.ext.KHR.h"
+        #include "GLSL.ext.EXT.h"
 #ifdef AMD_EXTENSIONS
         #include "GLSL.ext.AMD.h"
 #endif
@@ -351,6 +352,8 @@ const char* BuiltInString(int builtIn)
     case 5262: return "ViewportMaskPerViewNV";
 #endif
 
+    case 5264: return "FullyCoveredEXT";
+
     case BuiltInCeiling:
     default: return "Bad";
     }
@@ -862,6 +865,8 @@ const char* CapabilityString(int info)
     case 5260: return "PerViewAttributesNV";
 #endif
 
+    case 5265: return "FragmentFullyCoveredEXT";
+
     case CapabilityCeiling:
     default: return "Bad";
     }