Blender Git Loki

Sequencer: Transform ImBuf Processor.

Inside the sequencer the cropping and transform of images/buffers were
implemented locally. This reduced the optimizations that a compiler
could do and added confusing code styles. This patch adds
`IMB_transform` to reduce the confusion and increases compiler
optimizations as more code can be inlined and we can keep track of
indices inside the inner loop.

This increases end-user performance by 30% when playing back aa video
in VSE.

Reviewed By: ISS, zeddb

Differential Revision: https://developer.blender.org/D11549