Merge pull request #1467 from mbebenita/nat

Grab Bag
Parallelize `grunt shu` build target

Duration of a whole-system rebuild goes from ~37s to ~28s on my machine.
Merge pull request #1468 from tschneidereit/nat

Parallelize `grunt shu` build target
Check for zero or negative width/height arguments to bitmap data and throw appropriate errors.

There's a bug in Mining truck ATM where shapes are cached in zero by zero bitmap data objects because of bounds issues. This argument test exposes the bug early.