This fixes many of the Windows+GPU crashes.

Issues #96, #100, and more...

Changed when rays are normalized after being transformed to medium space
which will cause image differences with media that have a scale transform
applied to them. This fixes the visible volume majorant grid artifacts in
issue #28.

Otherwise small changes:
- The cloud medium's "extent" parameter has been renamed to "frequency".
- A small change to how it sums scales noise will lead to small image
diffs with it.
- Otherwise just renaming things / moving them around.