OfxEffectInstance: grouping is grouping!

- why not allow a « Time » plugin in the « Time » Grouping?
- RollingShutter needs no special treatment
- Keylight is now in the right grouping (Keyer, not Keyer/KeyLight)
EffectInstance: add questions about getRoD vs. getBounds

- remember that getBounds is the area where there is pixel data, and
getRoD is the full image (in canonical coords for now, but this may
change to pixel coords)