It was difficult to notice, but we weren't making use of the
`USDPrimReader::valid()` API calls during import.
In many(all?) cases this was fine as we would check the validity during
`read_object_data` or similar anyhow. Rather than just removing the API
entirely, this patch attempts to use it and has the following design:
- Where ever and whenever a reader is created, in addition to checking
null, we should now also check for `valid()` This happens in
`usd_capi_import` and `usd_reader_stage`.
- The `valid()` call is intended to check just the USD object status.
Blender object checks are handled elsewhere (same as they are
currently) since these objects are often not available at the time of
the call to `valid()`
This has the benefit that we at least know that USD is valid before our
heavy reading code ever starts executing. Some duplicate checks are now
removed.
Pull Request: https://projects.blender.org/blender/blender/pulls/129181
Reduce dependence on Blender headers as much as possible and move closer
to an include-what-you-use setup.
- Removes unnecessary includes
- Replaces some includes with more appropriate, narrower, substitutes
- Removes unnecessary forward declarations
Pull Request: https://projects.blender.org/blender/blender/pulls/118308
