glsl/ast: Forbid declaration of image variables in structures and uniform blocks.

glsl/ast: Forbid declaration of image variables in structures and uniform blocks.

Aggregating images inside uniform blocks is explicitly disallowed by
the standard, aggregating them inside structures is not (as of GL
4.4), but there is a similar problem as with atomic counters: image
uniform declarations require either a "writeonly" memory qualifier or
an explicit format qualifier, which are explicitly forbidden in
structure member declarations.  In the resolution of Khronos bug
#10903 the same wording applied to atomic counters was decided to mean
that they're not allowed inside structures -- Rejecting image member
declarations within structures seems the most reasonable option for
now.

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