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/test-preprocessor-list

@@ -4,6 +4,7 @@ preprocessor.edge_cases.vert
 preprocessor.errors.vert
 preprocessor.extensions.vert
 preprocessor.function_macro.vert
+preprocessor.include.vert
 preprocessor.line.vert
 preprocessor.line.frag
 preprocessor.pragma.vert