glsl: shader outputs cannot have initializers

glsl: shader outputs cannot have initializers

GLSL Spec 4.20.8, 4.3 Storage Qualifiers:

"Initializers in global declarations may only be used in declarations of
 global variables with no storage qualifier, with a const qualifier or
 with a uniform qualifier."

We do this for input variables, but not for output variables. AMD and NVIDIA
proprietary drivers don't allow this either.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>