This allows you to put any kind of animation data on the mesh, and its
shape will be exported on each timekey. Note that this timekey is unrelated
to the animation data (so we don't export on each keyframe, for example).
A practical example is the addition of an animated custom property to
trigger the export of animated mesh data. The mesh data can then be created
from any source, like Python scripts.
Not only is this useful in itself, it also provides a workaround for one
of the two issues described in T51351.
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