glsl: Handle centroid qualifier ordering in C code, not the parser.

glsl: Handle centroid qualifier ordering in C code, not the parser.

The GL_ARB_shading_language_420pack extension/GLSL 4.20 split centroid
off into a new category, "auxiliary storage qualifiers," and allow these
to be placed anywhere in the series.  So we have to stop recognizing
"centroid in"/"centroid out"/"centroid varying" in the grammar and get
more creative.

The same approach used before works here, too.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>