Add validation to VertexBuffer::build()
Add validation to VertexBuffer::build()

Clients should be prevented from allocating unused buffer slots because
uploading to an unused slot triggers undefined behavior in the backend.

Motivated by #2042

google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament