The existing point instancer reader is slightly refactored to allow for
animated setups. The primary change is simply to inherit from
`USDGeomReader` rather than `USDXformReader`. This allows access to the
`read_geometry` API used by the cache modifier.
The existing `read_object_data` method is split into two parts with
`read_geometry` loading per-frame USD data and `read_object_data`
coordinating the initial loading process, including creating the GN node
tree just once.
A new test has been added (a variation of the nested point instancer
file) with time samples on various attributes and on both point
instancers. This also fixes#129502 and the file provided in that issue.
----
The already added test file is `tests/data/usd/usd_point_instancer_anim.usda`
Pull Request: https://projects.blender.org/blender/blender/pulls/129881
Jesse Yurkovich
2523958e0e