Fixed bug #4837: "Knobs not found" warning messages.
The bug only appears when loading the LensDistort node UI for the first time with a file sequence specified. The cause was the early termination of the knob_changed() method before an update
to the dynamic knobs was made. The fix also the code simpler to read.
Fixed an issue where the artefacts were appearing around the edges of the bounding box despite the "black_outside" flag being turned on.
An additional result of this fix is that the distorted image is no longer clipped slightly around the edges.
Merge pull request #143 from goddardl/LensDistortMissingKnobError

Fixed bug #4837: "Knobs not found" warning messages.