Make bitmap-ref and flex-bitmap-ref tests work in the reftest harness

They both were set to take a snapshot of frame 0, but are only fully initialized on frame 1. While this changed from before-nat master, I don't think it's really a regression: attaching bitmaps is an asynchronous operation.
Disable colors in Grunt if output is redirected to a pipe or file

Should make our botio logs much nicer.
Merge pull request #1610 from tschneidereit/master

Make bitmap-ref and flex-bitmap-ref tests work in the reftest harness