Added undef intrinsic.

Stores that would store an undef are skipped. This lets you write in-place
updates to an output buffer and functions with update steps and no pure steps.

See the two new tests (undef and in_place) for example usage.
Fix for loading grayscale pngs using Halide's Image class (not static_image)
Merge branch 'master' of github.com:halide/Halide