Add multiple texture support, improve compatibility

Add initial support for supporting multiple textures embedded in a gltf / glb. I have tested a small number of multiple texture models and they work, but if you have one that does not, please send me a link to the glb file so I can try it out. In the picture these models use on texture for the bodies and another for their held items.

Performance note: it is still better to only use a single texture for performance, whenever the renderer needs to change textures, it lowers the efficiency of the C3 renderer and the GPU. So, generally, if you come across a model with multiple textures, explore the video which describes how to bake down to one texture.

Another performance note: high polygon models with animation and rotation require the CPU (not GPU) to do a lot of matrix math calculations and can hurt performance, so look at using lower polygon models, decimating high polygon models or reducing the animation rate or rotation rate (how often to rotate the model.)

I also removed some more 'modern' JS features from the addon, in an attempt to improve compatibility with older browsers/platforms.

The example included in this devlog shows an example of loading / creating a model at runtime. Type one of the project file names into text box and press load, to create and load a new instance. You can drag and drop the models. Right-click to delete a model. Use the mouse scroll wheel to change the model scale.

With the RPG character models, I still have an issue that I am debugging regarding rotating on two or three axes (different meshes rotate differently.)

Enjoy!