glsl: implement ARB_bindless_texture conversions

glsl: implement ARB_bindless_texture conversions

From section 5.4.1 of the ARB_bindless_texture spec:

   "In the following four constructors, the low 32 bits of the
    sampler type correspond to the .x component of the uvec2 and
    the high 32 bits correspond to the .y component."

    uvec2(any sampler type)     // Converts a sampler type to a
                                //   pair of 32-bit unsigned integers
    any sampler type(uvec2)     // Converts a pair of 32-bit unsigned integers to
                                //   a sampler type
    uvec2(any image type)       // Converts an image type to a
                                //   pair of 32-bit unsigned integers
    any image type(uvec2)       // Converts a pair of 32-bit unsigned integers to
                                //   an image type

v4: - fix up comment style
v3: - rebase (and remove (sampler) ? 1 : vector_elements)

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