glsl: Do not allow assignments to read-only buffer variables

glsl: Do not allow assignments to read-only buffer variables

v2:
  - Merge the error check for the readonly qualifier with the already
    existing check for variables flagged as readonly (Timothy).
  - Limit the check to buffer variables, image variables have different
    semantics involved (Curro).

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