Blender Git Loki

Fix T75574: Instant crash: averaging custom normals on model.

Fairly critical code mistake actually, since it uses malloca,
BLI_smallstack should *never* be declared inside a loop...

Also optimized handling of the `loop_weight` heapsimple, we can also
only create and use a single one for all edited objects...

Found two other operators potentially affected by same issue (split
normals, and weld edges into faces).