Blender Git Loki

Two in one:

- Bugfix #25937
Child-of constraint now behaves like regular parent-child
relationship when all options are set. This prevents the
errors that can happen when decomposing non-uniform matrices.

- Todo item
The area corner hotspots for splitting/merging were far too
narrow. Now it uses a circular distance to detect whether
the hotspot is active. Also cleaned up drawing code for it.