Use multi-threading within Encoder.py to speed up compression time

[tim-depthkit]

It would be nice to have the option to run the encoder with multiple threads in order to run the draco_encoder and basisu subprocesses in parallel. This could be accomplished with the python threading module.

[speigg]

Hi Tim! Thanks for opening the issue! Yes, we will be working towards full-featured encoding software that efficiently encodes to multiple targets, including both Geometry and Texture LODs generated from source assets.