Add #include processing to glslang (though turned off by default).

When an include directive is recognized by the preprocessor, it
executes a callback on the filepath argument to obtain the file
contents.  That way the compilation client can deal with the file
system, include paths, etc.

Currently only accepts quoted filepaths -- no angle brackets yet.

glslang/MachineIndependent/preprocessor/PpAtom.cpp

@@ -120,6 +120,9 @@ const struct {
     { PpAtomLineMacro,       "__LINE__" },
     { PpAtomFileMacro,       "__FILE__" },
     { PpAtomVersionMacro,    "__VERSION__" },
+
+    { PpAtomInclude,        "include" },
+
 };
 
 } // end anonymous namespace