Blender Git Loki

Fix T49893: Crash in Video Sequence Editor with 'drop' effect.

Code was not accounting for possibilities that width or height of given
buffers may be smaller than XOFF/YOFF...

Note that I seriously doubt that drop code actually works (as in, gives
expected results) when applied to tiles like it seems to be done
currently, but this is much more complex (and involved) topic.