Added Math.previousPowerOfTwo

[IanLilleyT]

This PR adds Math.previousPowerOfTwo (Math.nextPowerOfTwo already exists)

Also improved the docs and specs to handle out of range inputs since Javascript bitwise operations convert operands to signed 32-bit int

[cesium-concierge]

Thanks for the pull request @IanLilleyT!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[lilleyse]

Looks good, thanks @IanLilleyT

[lilleyse]

There are a few PRs including this that need updates to CHANGES.md. @IanLilleyT's going to push that to master.