glsl: Refactor code to check that identifier names are valid.

glsl: Refactor code to check that identifier names are valid.

GLSL reserves identifiers beginning with "gl_" or containing "__", but
we haven't been consistent about enforcing this rule.  This patch
makes a new function to check whether identifier names are valid.  In
the process it closes a loophole where we would previously allow
function argument names to contain "__".

v2: Rename check_valid_identifier() -> validate_identifier().  Add
curly braces in validate_identifier().

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>