Panda3d-gltf

Converter
* Fix error when no top-level `extensions` object is present
* Fix crash when using Panda3D compiled with double-precision floats
* Add support for emission maps
* Fix direction and lens view vector for directional lights
* Do not assign empty TransformBlendTable if a Geom does not have a `transform_blend` column

Viewer
* Improved default camera position and view
* Add work-around to make lights from the model work
* Upgrade to `panda3d-simplepbr>=0.6` for emission and occlusion map support
* Shortcuts added to toggle various render features:
* n: toggle normal maps
* e: toggle emission maps
* o: toggle occlusion maps
* a: toggle ambient light

Other
* Add `--version` flag to CLI
* Use `GltfLoader.global_settings` if `gltf_settings` is `None` for `patch_loader()`

Viewer
* Toggle wireframe view with `w`
* Toggle textures with `t`
* Dump scene graph to standard out with `shift-l`
* Dump analysis of the scene graph to standard out with `shift-a`

Converter
* Fix an issue where morph targets would no longer be bound after saving and loading to model cache

Converter
* Add `--no-srgb` option to skip making sRGB textures (not spec-complaint, but may be useful for some workflows)
* Add options to copy textures
* Add support for morph targets and animations
* Add support for multiple meshes using the same skin
* Add support for skeletons with multiple root bones
* Fix some transformation issues with characters
* Fix color of fallback normal map
* Fix texture stage order (make it deterministic)
* Fix fallback textures not getting saved to BAM files

Many thanks to el-dee and rdb who did most of the work for this release!

Converter
* Faster loading of bin files
* Support loading supplied tangents
* Calculate tangents if they are missing
* NOTE: this is still WIP and not spec-compliant
* Support loading normal maps
* More axis-conversion fixes

Viewer
* The default camera will now look at the loaded model

Viewer
* Move rendering and shaders to a new, separate [panda3d-simplepbr](https://github.com/Moguri/panda3d-simplepbr)
* Updates from panda3d-simplepbr:
* Better support for OpenGL 2.1 hardware
* Multi-sample Anti-aliasing

Conversion
* Initial support for KHR_lights_punctual extension
* Fix bad file paths for Python 3.5
* Fix bad file paths for Windows

Viewer
* Do not load models from the model cache
* Support OpenGL 2.1 hardware

Other
* Register glb as a file extension for the Python loader