This is a revamped equivalent to #9902, though it shares none of the
code. It handles all special cases that I've tested correctly.
The overall technique consists of double-buffering the joint matrix and
morph weights buffers, as most of the previous attempts to solve this
problem did. The process is generally straightforward. Note that, to
avoid regressing the ability of mesh extraction, skin extraction, and
morph target extraction to run in parallel, I had to add a new system to
rendering, `set_mesh_motion_vector_flags`. The comment there explains
the details; it generally runs very quickly.
I've tested this with modified versions of the `animated_fox`,
`morph_targets`, and `many_foxes` examples that add TAA, and the patch
works. To avoid bloating those examples, I didn't add switches for TAA
to them.
Addresses points (1) and (2) of #8423.
## Changelog
### Fixed
* Motion vectors, and therefore TAA, are now supported for meshes with
skins and/or morph targets.
be053b1d7c