glsl: Add support for constant expression evaluation on round(), roundEven().

glsl: Add support for constant expression evaluation on round(), roundEven().

v2: Avoid the C99 rounding functions, because I don't trust
get/setting the C99 rounding mode from inside our library not having
other side effects.  Instead, open-code roundEven() behavior around
Mesa's IROUND, which we're already testing for C99 rounding mode
safety.

Fixes glsl-1.30/compiler/built-in-functions/round*

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