* gltfio: Add internal MorphHelper, enable up to 255 targets.
Previously, gltfio could not animate models with more than 4 morph
targets. Now we can handle up to 255 targets, as long as no more than
4 targets are ever used simultaneously. This allows us to handle
many more models, such as those exported by Modo, and the horse model
in #1852.
We do this by picking the highest 4 weights. The indices for these
weights are called the "primary indices", and we construct a new
VertexBuffer for each unique set of primary indices.
In the near future this will be made more efficient by adding a new
VertexBuffer API to the Filament core.
Fixes#1487.
* Code review fixups.
8d78b1f2e1