The spec has been updated so that `requestPresent` can accept empty arrays for `leftBounds` and `rightBounds`. Though Chromium returns `null` for these properties by default, Firefox returns empty arrays, and there is a proposal to make those non-nullable. This patch should fix rendering for Firefox, which is currently broken.

For reference:
https://github.com/w3c/webvr/issues/95
https://github.com/w3c/webvr/issues/89

They didn't produce correct eye translations in all cases (specifically
standing scenes.) This reverted version may not be 100% accurate on
future hardware (if the eyes have a rotation component, for example) but
it's correct on all current WebVR implementations. That's better than
blatantly wrong in some cases.

Use of the WebVR-provided projection matricies has been left in, since
that was working correctly.

Replaced calls to recently removed methods (.GetSrcObjectCount and
.GetSrcObject, previously deprecated) with existing variants.
See
http://help.autodesk.com/view/FBX/2017/ENU/?guid=__files_GUID_2A50CD0F_062D_4F80_88C7_BBA0483FBE0F_htm
for details

* remove repeated calculation from renderer

* remove now redundant getter