Blender Git Loki

Performance: Use parallel range for ImBuf scanline processor.

Scanline processor did its own heurestic what didn't scale well when
having a multiple cores. In stead of using our own code this patch will
leave it to TBB to determine how to split the scanlines over the
available threads.

Performance of the IMB_transform before this change was 0.002123s, with
this change 0.001601s. This change increases performance in other areas
as well including color management conversions.

Reviewed By: zeddb

Differential Revision: https://developer.blender.org/D11578