fix vendor prefix checks, closes #218 and #208

Stuff fixed:

- ElementOutput required that document.body exists, which makes this not work when added in head and also if there is no body.
- Context was adding two style declarations (unprefixed and prefixed) even when they weren't needed. This will detect the need for vendor prefixes to reduce the total number of DOM hits on perspective change
- Context was including an invalid value on non-prefixed perspective style declarations (`1000` instead of `1000px`

This will solve all of that