Cursor: Fix Zoom and Grab prefixing ranges

- Firefox only unprefixed the zoom properties in 24, not the grab
properties
- Chrome supported the zoom properties in 3 as well, but the grab is
correct that it didn't support them till 4
- Typos in comments
Merge pull request #247 from nschonni/cursor-vendor-prefix-values

Cursor: Fix Zoom and Grab prefixing ranges