Blender Git Loki

Fix T61226: Make hair editing (more?) thread safe

This replaces the global `PEData->dist` with a thread local variable
that is passed into the callback functions separately.

There are two new function types which have `Hit` in the name.
Those take the distance to the mouse as an additional parameter.

Reviewers: sergey

Differential Revision: https://developer.blender.org/D4538