Blender Git Loki

Fix T61234 Mirroring Grease Pencil keyframes in the Dopesheet fails

`ANIM_animdata_update()` did not sort grease pencil frames. A
pre-existing comment stated this wouldn't be necessary as
`posttrans_gpd_clean()` already does this. However, this is only
applicable when the change is performed via the transform system. The
mirror operator doesn't call `posttrans_gpd_clean()`, invalidating the
assumption in the comment.

I moved the sorting code into `BKE_gpencil_layer_frames_sort()`, which
is now called from both `ANIM_animdata_update()` and
`posttrans_gpd_clean()`.