glsl: do not make sampler/image types readonly variables

glsl: do not make sampler/image types readonly variables

In plain GLSL, sampler and image types can only be declared
uniform-qualified global variables or 'in' function parameters.

Setting the read_only flag seems quite useless because other
checks will prevent sampler/image variables to be assigned and
also because the flag is not set for atomic_uint types which are
opaque types.

This will also help for ARB_bindless_texture because samplers
and images can be assigned when they are considered bindless.

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