Panda3d-gltf

* 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

Converter
* Do a better job combining associated GeomNodes under Characters
* Fix loading textures from cache (fixes 118)

CLI
* Fix paths on Windows

Converter
* Stop using `load_prc_file_data()` (fixes 109)
* Make GltfSettings a mutable dataclass
* Remove `gltf.patch_loader()`
* Support `KHR_materials_ior`, set default refractive index to 1.5
* Make inverseBindMatrices optional on skins (fixes 111)
* Remove `convert()` function
* Rename `GltfSettings.animations` to `GltfSettings.skip_animations` (defaults to `False`)
* Remove `writing_bam`/`copy_lights` hack (we no longer support the older Panda3D versions that required these)
* Stop using `model-path` to find/load textures (fixes 115)
* Stop writing embedded textures to disk (this was in addition to being embedded in the BAM file)
* Remove `outdir` and replace `indir` with `filepath/filedir`
* Add `flatten_nodes` option that attempts to flatten the output node hierarchy (fixes 101)
* Handle case where glTF `extras` is not an object/dict (fixes 112)
* Put morph targets on separate `GeomVertexArray` (fixes 117)

Loader
* Stop using an intermediate BAM file (fixes 116)
* Load settings from ConfigVars and remove `GltfLoader.global_settings`

CLI
* Show defaults in help/usage text
* Add `--collision-shapes` option to replace `--physics-engine`, which is now deprecated (fixes 100)
* Convert `src` and `dst` to absolute paths

Other
* Move `load_model()` from converter to loader (still available as `gltf.load_model()`)