PickId refactoring / cleanup.

* pickObjects is now per-context instead of global.
* Color now has toRgba/fromRgba methods to convert to/from 32-bit RGBA values.
* pickObjects is now indexed by RGBA numeric color values.
* PickId now has a single `color` property of type Color.
* Removed use of "unnormalized" (i.e. invalid) values in Color instances.
* Context.getObjectByPickId renamed to getObjectByPickColor.
Update CHANGES, other tweaks.
Fix doc typo.
Merge pull request #647 from AnalyticalGraphicsInc/commandPassState-pickIdChanges

PickId refactoring / cleanup.