glsl: Add support for `precise` redeclarations

This works like glsl-1.20+'s invariant redeclarations, but with fewer
restrictions, since `precise` is allowed on pretty much anything.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
4 files changed