Added a failing test case to GafferImageTest.ImageTransformTest that highlights a bug that has
been introduced since the node was first written.
Fixed bug in ImageTransform.

This was caused by ImageTransform::hashChannelData() not correctly including the tile origin in the hash. For transforms where two output tiles share the exact same set of input tiles affecting them, we were computing exactly the same hash for both output tiles, even though they each would filter the pixels in the input tiles totally differently.

This bug was introduced in 78b8739f1867ecd3b660838d384cdd6083734d92, where I refactored the hash computation for images. Specifically, in that commit it became the responsibility of derived classes to hash in the tile origin where necessary whereas before the SceneProcessor _always_ hashed in the tile origin whether it was necessary or not. The old code resulted in unnecessary computation for networks like Constant->Grade, where although all the tiles have the same hash in the Constant, they would have different hashes in the Grade. The new way of doing things is for sure better, provided you're not a numpty like I was and fail to hash in the tile origin where relevant.

Also renamed the test case, so it actually makes sense now we've fixed things - in any case we should have used @GafferTest.expectedFailure to decorate it in the first place so we could still give it a meaningful name.
Simplified ImageTransform::channelDataHash() and removed todo.

Using the less verbose method of hashing all the children of a CompoundPlug.

The comment in the todo was inaccurate - ImageProcessor no longer does any hashing of the tile origin. We also won't be changing the coordinate space that tile origin is specified in - tile origin, display window, and data window are all specified relative to the same origin for good reasons - sanity and ease of understanding. There's no reason the described optimisation couldn't work with the origin we have, but it seems best to implement in a node which specifically does axis-aligned integer shifts in the image without any filtering.
Merge branch 'goddardl-ImageTransformBug'