glsl: Do not allow reads from write-only buffer variables

glsl: Do not allow reads from write-only buffer variables

The error location won't be right, but fixing that would require to check
for this as we process each type of AST node that can involve a variable
read.

v2:
  - Limit the check to buffer variables, image variables have different
    semantics involved.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>