GLSL: Emit better diagnostics when accessing ImageParam w/o explicit bounds

GLSL code generation requires that color indices are compile-time constants,
which causes problems when using ImageParams without explicit constraints on min[2].

This patch uses min[2]==0 (the most common case) if no explicit constraints are given
and also emits a warning.

I'm undecided whether using a min[2]=0 as a default is a good idea, but since it's a problem every user of the GLSL backend will eventually run into, I think it may be... Opinions?