Blender Git Loki

T78995: Enable keylist threaded drawing.

This enabled multithreaded building of the keys that needs to be drawn
in the timeline (and other action editors).

On an AMD Ryzen 3800 using a mocap data test file (available in patch)
the performance went from 2fps to 8fps. The performance increase depends
on the number of rows of keyframes that is shown in for example the
timeline editor.

Each row will be using a different thread. Currently the bottleneck is
the summary channel that we could split up in the future even more (
although that is a complex refactoring work).

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D12198