glsl: allow bindless samplers/images to be used with constructors

glsl: allow bindless samplers/images to be used with constructors

For the explicit conversions.

From section 4.1.7 of the ARB_bindless_texture spec:

   "Samplers are represented using 64-bit integer handles, and
    may be converted to and from 64-bit integers using constructors."

From section 4.1.X of the ARB_bindless_texture spec:

   "Images are represented using 64-bit integer handles, and
    may be converted to and from 64-bit integers using constructors."

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> (v2)
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>