glsl: allow precision qualifier on sampler arrays

glsl: allow precision qualifier on sampler arrays

If a precision qualifer is allowed on type T, it should be allowed
on an array of T.  Refactor the check to ensure this is the case.

(Fixes failures in WebGL conformance test 'gl-min-textures')

Signed-off-by: Frank Henigman <fjhenigman@google.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>