glsl: allow bindless samplers/images as varying variables

glsl: allow bindless samplers/images as varying variables

From section 4.3.4 of the ARB_bindless_texture spec:

   "(modify third paragraph of the section to allow sampler and image
    types) ...  Vertex shader inputs can only be float,
    single-precision floating-point scalars, single-precision
    floating-point vectors, matrices, signed and unsigned integers
    and integer vectors, sampler and image types."

From section 4.3.6 of the ARB_bindless_texture spec:

   "Output variables can only be floating-point scalars,
    floating-point vectors, matrices, signed or unsigned integers or
    integer vectors, sampler or image types, or arrays or structures
    of any these."

v3: - add spec comment

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