Panda3d-simplepbr

* Fix finding shaders on macOS
* Fix warnings and errors when using GLSL 330 shaders
* Use semantic texture slots (texture stages) instead of texture indices

* Add light attenuation
* Allow updating `render_node`, `window`, and `camera_node` properties after setup
* Add `use_330` option to control using GLSL 330 shaders instead of GLSL 120
* Support texture transforms
* Add hardware skinning
* Enabled by default on OpenGL 3.2+ capable hardware
* Can be controlled with `use_hardware_skinning` option

**Note:** Panda3D 1.10.8+ is now required

* Fixes for shader/rendering artifacts
* Clamp NdotL and NdotV to 0.0
* Fix GGX/Trowbridge-Reitz equation for normal distribution function
* Fix normalization of normal vector in the fragment shader
* Optimizations
* Use a dedicated shadow shader to avoid expensive PBR functions (mostly helps Intel)
* Remove Fresnel term from diffuse (i.e., use simple Lambertian)
* Use Spherical Gaussian approximation in the Fresnel equation
* Automatically enable auto shader on TextNodes found in the scene graph (fixes TextNode rendering)

* Ensure AntialiasAttrib is set on the render NodePath (ensures MSAA works if enabled)
* Support occlusion maps in the red channel of an occlusion-metal-roughness (ORM) map
* Fix ambient lights
* Support emission maps
* Shaders moved to their own directory

* Name structs to improve compatibility with Intel drivers
* Support p3d_ColorScale (modifies the base color)
* Allow users to set an exposure value
* Allow changing max_lights, use_normal_maps, msaa_samples, and exposure at runtime
* Add basic shadow map support for DirectionalLight and Spotlight (NOTE: This breaks if a PointLight is present, and must be explicitly enabled)
* Add exponential fog (must be explicitly enabled since fog density does not default to 0)

* Add support for vertex colors (from rdb)