ParabolicLineCast's maximum length is dependent on rotation

The current parabolic pointer implementation (`ParabolicLineCast`) inherits the existing problem with the bezier pointer cast. Images courtesy of @thestonefox.

Normal cast:

Limited cast:
(Down doesnt reach floor so not valid target)

Simply angling the forward cast beam down means you can now reach the floor

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There is an open bug for this issue:
https://github.com/thestonefox/VRTK/issues/1288

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No viable solution as of yet, but here's some ideas:
- Maybe it relies on rotation of beam? Or always does a forward beam in world forward for working out viability of the downward length?
- Don't limit the downward beam. Instead you limit the forward beam and infinitely cast the down ray, then cast a 3rd ray between origin and down rayhit, and that 3rd ray is what you limit?