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.
llvm default argument issue fix for os x

private scope copy constructor was preventing using
a scope as a default arg even when the arg type
was a const ref
Merge pull request #380 from halide/glsl_constraints

GLSL: Emit better diagnostics when accessing ImageParam w/o explicit bounds
Tell llvm we expect assertions to be true.
Merge branch 'master' of github.com:halide/Halide
Express likely branches more robustly.

The previous solution only worked for trunk llvm.
Fixed a few remaining instances passing temporary Scope objects to const references.
Merge branch 'master' into glsl_codegen