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 constraints are given
and emits a warning.
Merge pull request #380 from halide/glsl_constraints

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