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/PpTokens.h

@@ -158,6 +158,9 @@ enum EFixedAtoms {
     PpAtomFileMacro,
     PpAtomVersionMacro,
 
+    // #include
+    PpAtomInclude,
+
     PpAtomLast,
 };