glsl: allow bindless samplers/images to be initialized

glsl: allow bindless samplers/images to be initialized

From section 4.1.7 of the ARB_bindless_texture spec:

   "Samplers may be declared as shader inputs and outputs, as uniform
    variables, as temporary variables, and as function parameters."

From section 4.1.X of the ARB_bindless_texture spec:

   "Images may be declared as shader inputs and outputs, as uniform
    variables, as temporary variables, and as function parameters."

v3: - add spec comment
    - update the glsl error message

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