glcpp: Allow vertical tab and form feed characters in GLSL

glcpp: Allow vertical tab and form feed characters in GLSL

Of course, these aren't really useful for anything, but the GLSL language
specification does allow them:

	The source character set used for the OpenGL shading languages,
	outside of comments, is a subset of UTF-8. It includes the following
	characters:
	...

	White space: the space character, horizontal tab, vertical tab, form
	feed, carriage-return, and line- feed.

	[GLSL Language Specification 4.30.6, section 3.1]

So treat vertical tab ('\v' or ^K) and form-feed ('\f' or ^L) as horizontal
space characters.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>