glsl: Add image type to the GLSL IR.

glsl: Add image type to the GLSL IR.

v2: Reuse the glsl_sampler_dim enum for images.  Reuse the
    glsl_type::sampler_* fields instead of creating new ones specific
    to image types.  Reuse the same constructor as for samplers adding
    a new 'base_type' argument.

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