glsl: reject bindless samplers/images frag inputs without 'flat'

glsl: reject bindless samplers/images frag inputs without 'flat'

From section 4.3.4 of the ARB_bindless_texture spec

   "(modify last paragraph, p. 35, allowing samplers and images as
    fragment shader inputs) ... Fragment inputs can only be signed
    and unsigned integers and integer vectors, floating point scalars,
    floating-point vectors, matrices, sampler and image types, or
    arrays or structures of these.  Fragment shader inputs that are
    signed or unsigned integers, integer vectors, or any
    double-precision floating- point type, or any sampler or image
    type must be qualified with the interpolation qualifier "flat"."

v3: - update spec comment formatting

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