glsl: allow bindless samplers/images to be l-values

glsl: allow bindless samplers/images to be l-values

From section 4.1.7 of the ARB_bindless_texture spec:

   "Samplers can be used as l-values, so can be assigned into and
   used as "out" and "inout" function parameters."

From section 4.1.X of the ARB_bindless_texture spec:

   "Images can be used as l-values, so can be assigned into and
    used as "out" and "inout" function parameters."

v4: - invert the logic
v3: - update spec comment formatting
    - keep the read_only check

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>