This replaces header include guards with `#pragma once`.
A couple of include guards are not removed yet (e.g. `__RNA_TYPES_H__`),
because they are used in other places.
This patch has been generated by P1561 followed by `make format`.
Differential Revision: https://developer.blender.org/D8466
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
