This was caused by a double lock of the DRW context mutex.
This changes the logic a bit by releasing the DRW context before rendering
with BKE_sequencer_give_ibuf and restoring it after.
Critical fix for 2.91
Reviewed By: dfelinto
Differential Revision: https://developer.blender.org/D8657
Move sample operator functions to `ed_util_imbuf.c` and change common functions,
so they can be used in image editor and sequencer.
Reviewed By: campbellbarton
Differential Revision: https://developer.blender.org/D7315
