Bezier handles are recalculated in many places in the
animation code. Threading that code can give a performance
boost all over Blender.
This patch only threads a part of the handle calculation code.
`BKE_nurb_handle_smooth_fcurve` can still be run in parallel,
but that's more complicated, so not done in this PR.
Overall this patch mostly benefits code paths that are not already threaded.
------
Performance delta
| Action | Before | After |
| - | - | - |
| `recalcData_graphedit` moving a single key | 1.06 ms | 1.0 ms |
| `recalcData_graphedit` moving 300 keys of a single FCurve | 1.6 ms | 1.4 ms |
| `recalcData_graphedit` moving 300 keys of multiple FCurves | 60 ms | 55 ms |
| `ANIM_animdata_update` when using the Breakdown operator in the GE | 90 ms | 73 ms |
Test file used
https://download.blender.org/ftp/sybren/animation-rigging/heavy_mocap_test.blend
Pull Request: https://projects.blender.org/blender/blender/pulls/119388
afe13175da