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.

Test/baseResults/preprocessor.include.vert.err

@@ -0,0 +1,8 @@
+ERROR: 0:8000: '#include' : must be followed by a file designation 
+ERROR: 0:8001: '#include' : must be followed by a file designation 
+ERROR: 0:8002: '#error' : unexpected include directive  
+ERROR: 0:8003: '#include' : extra content after file designation 
+ERROR: 0:8004: '#error' : unexpected include directive  
+ERROR: 5 compilation errors.  No code generated.
+
+