Update ReflectionManager on Scene Field Change

During side-by-side rendering the refraction texture needs to be updated
for both the left and right fields.  These changes add a new GFXDevice
event type to track when a field is about to be rendered.  The
ReflectionManager listens to this new event and ensures that the
refraction texture will be updated if it is referenced by a material.
Merge pull request #516 from DavidWyand-GG/OculusRiftUpdate2

Update ReflectionManager on Scene Field Change