glsl: make sampler/image types as 64-bit

glsl: make sampler/image types as 64-bit

The ARB_bindless_texture spec says:

   "Samplers are represented using 64-bit integer handles."

and,

   "Images are represented using 64-bit integer handles."

It seems simpler to always consider sampler and image types
as 64-bit unsigned integer.

This introduces a temporary workaround in _mesa_get_uniform()
because at this point no flag are used to distinguish between
bound and bindless samplers. This is going to be removed in a
separate series. This avoids breaking arb_shader_image_load_store-state.

v3: - update the comment slightly

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