glsl: Add helper methods to glsl_type for dealing with images.

glsl: Add helper methods to glsl_type for dealing with images.

Add predicates to query if a GLSL type is or contains an image.
Rename sampler_coordinate_components() to coordinate_components().

v2: Use assert instead of unreachable.
v3: No need to use a separate code-path for images in
    coordinate_components() after merging image and sampler fields in
    the glsl_type structure.

Reviewed-by: Paul Berry <stereotype441@gmail.com>