ID 156186 - VectorBlur ignores image data outside of the image format in "forward" mode

VectorBlur nodes in "forward" mode ignore any data outside of the image format even if the bounding box is larger than the image, so it effectively crops the image.

Backward mode on the other hand retains data outside of the image format and blurs it as expected.

The attached example script demonstrates the problem. The ScanlineRender node has overscan enabled to create a large bbox than the image format. The 3 different VectorBlur nodes then give different output

- VectorBlur1 uses the "backward" method and outputs all channels. The whole image is blurred as expected

- VectorBlur2 uses the "forward" method and outputs just rgba. The data outside of the image format is just the edge pixel stretched out.

- VectorBlur3 uses the "backward" method and outputs to all channels. The data outside of the image format is blank so the bbox has been ignored.

I've reproduced this with Nuke 7.0v1, 7.0v10 and 9.0v4 on Windows and with Nuke 9.0v4 on OSX and Linux (I've not checked earlier versions) and the behaviour is the same on all platforms and versions.