Panda3d-gltf

Converter
* Handle physically-based units for punctual lights
* Stop adding fallback textures
* Attempt to re-use texture stages
* Use correct texcoord for TextureStage hash if KHR_texture_transform is used

* viewer: Fix error from missing "gltf/hdri/512_cg_0.hdr" file

* converter: Re-sample animations on import instead of assuming one data value per frame (note: CUBICSPLINE is not supported yet)
* converter: Allow non-standard absolute paths in image URIs

* converter: Fix urllib.parse import
* converter: Save space when storing animation data when all frames for a channel have the same value

Converter
* Fix skinned meshes with a transformed root node

Converter
* Fix error when a collision shape's bounding box contains a zero dimension
* Fix loading textures with spaces in the file path

CLI
* Fix paths when copying textures
* Make `dst` option (use `src` with `.bam` extension as the default)
* Remove deprecated `--physics-engine` flag

Other
* Re-add Python 3.8 support
* `gltf.converter` and `gltf.loader` are now explicitly marked private