glsl: Add API to put default precision qualifiers in the symbol table

glsl: Add API to put default precision qualifiers in the symbol table

These have scoping rules that match the ones defined for other things such
as variables, so we want them in the symbol table.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>