* move the focus distance from DofOptions to Camera
The DofOption parameter is now deprecated, but will still work if the
focus distance is not set on Camera.
* A few new helper APIs on Camera
- getFocalLength() which returns the focal length used internally for
DoF computations.
- computeEffectiveFov() and computeEffectiveFocalLength() which can be
used to better simulate a real camera's FOV changes with the
focus distance.
Modified gltf_viewer so it uses the effective fov/focallendth when
DoF is active.
* simplify setLensProjection
We can call setProjection directly instead of converting to a fov
first, which ends up performing atan(tan(x)).
Also improve parameters names.
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