This moves our existing VertexBuffer utility into a new "geometry"
library and adds new functionality for computing tangents based on UV's.
The UV-based method comes from Eric Lengyel.
We considered mikktspace (which thankfully has a zlib-style license) but
it would require re-indexing via meshoptimizer and is therefore a bit
heavyweight.
The new library has no dependencies and will add only 7 KB to Filament's
Android aar file.
Fixes#858 and preps for #528.
