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.

This patch introduces a new extension, GL_GOOGLE_include_directive,
to enable support #include directives. It depends on the extension
GL_GOOGLE_cpp_style_line_directive.

After parsing a #include directive, we push a TokenizableString
which contains the content of the included file into the input
stack. Henceforth, tokens will be read from the newly pushed
TokenizableString. However, the scanner in TParseContext still
points to the previous input stream. We need to update the scanner
to point to the new input stream inside TokenizableString. Thus,
the setCurrent{String|Line|..} method in TParseContext updates
the status of the correct input stream. After finishing the newly
pushed TokenizableString, we need to restore the scanner to the
previous input stream.

Also changed the includer interface to let it return the actual
full path of the included file.

Now extensions required by preprocessor should be checked via
the ppRequireExtensions method. This is more clear and coherent
with the rest of the code.