Parallelize the implementation and move constant checks out of hot
loops. Only push undo steps for PBVH nodes that actually have changed
hide status. Optimize the "show all" case to remove the hide attributes.
For timings, I recorded some information on a 16 million vertex mesh:
- Hide masked (1/4 of mesh): 171 ms to 33 ms
- Hide small box: 13 ms to 14 ms
- Show all with small box hidden: 226 ms to 10 ms
- Show with all visible: 173 ms to 0.36 ms
There are a few other visbility operators that aren't affected:
- SCULPT_OT_face_set_invert_visibility
- SCULPT_OT_reveal_all
- SCULPT_OT_face_set_change_visibility
In separate steps, they should be moved to use the same structure, or
maybe even removed in the case of "reveal all".
I expect the "gather, change hidden verts, scatter" steps for each node
could be made a bit more efficient, they do some redundant work.
But it was also a simple way to share a reasonable amount of code.
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