glsl: Allow precision qualifiers on general opaque types.
glsl: Allow precision qualifiers on general opaque types.

From the GLSL ES 3.1 spec, section 4.7.3:
 "Any floating point, integer, opaque type declaration can have the
  type preceded by one of these precision qualifiers: [...] highp
  [...], mediump [...], lowp [...]."

Reviewed-by: Timothy Arceri <7f85c8176b2576ffafd04ac63cd29ed35ff86cb6@yahoo.com.au>
Reviewed-by: Tapani Pälli <cdcb67a5e49297de94340ca09f25994bd5e7f762@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler