glTF models, draco compression

[pierotofy]

Adds support for generating binary glTF textured models (GLB), with draco and JPEG compression.

Enabled via --gltf.

Supersedes OpenDroneMap/WebODM#1283

[pierotofy]

[pierotofy]

TODO:

• Convert multispectral texture data to 8bit

[TimeScience]

GLTF models can't be ingested by Sketchfab. I've tried two very different models so I'm concluding this is an ODM issue not something particular to one model.
They also all fail when opening in Meshlab with the error: "accessor[0] invalid bufferView"
@pierotofy, can you have a look?
See for example this model: https://asdc.cloud.edu.au/public/task/6b947e2e-ea34-4fb9-b8ac-3e837cff3758/3d/