glsl: Change the parser to use the string buffer

glsl: Change the parser to use the string buffer

Signed-off-by: Thomas Helland <thomashelland90@gmail.com>
Tested-by: Dieter Nützel <Dieter at nuetzel-hh.de>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle at amd.com>
V2: Pointed out by Timothy
   - Fix pp.c reralloc size issue and comment

V3 - Use vprintf instead of printf where we should
   - Fixes failing make-check tests

V4 - Use buffer_append_char in a couple places
   - Use append_char in even more places