Do not require window.ENV for DISABLE_RANGE_API.

Prioritizes `MetamorphENV` over `ENV` to disable the range API, allowing the
usage of the `ENV` global for other purposes.

This is fully backwards compatible and will simply fall back on `ENV`.

I would have preferred to use `Metamorph.ENV.DISABLE_RANGE_API` as the flag,
but the `var Metamorph` overrides (due to hoisting) any global setup at
`Metamorph` outside the closure.