Adds a Point Instancing exporter based on the existing
USDPointInstancerReader. Covers both round-trip and Blender-native
workflows. Exports 'Instance on Points' setups as USDGeomPointInstancer,
supporting objects, collections, and nested prototypes.
A warning is shown during export if invalid prototype references are
detected. These would occur if an instancer attempts to instance itself.
This feature is currently gated behind an off-by-default export option
(`use_instancing`) as there are still a few cases which can yield
incorrect results.
Further details in the PR.
Ref: #139758
Authored by Apple: Zili (Liz) Zhou
Pull Request: https://projects.blender.org/blender/blender/pulls/139760
07342407d3