reduce bandwidth in vertex data upload to gpu

[capnramses]

i need a hand here - this is the bit i referred to in the PR description. i assumed i could just not upload indices/uvs if it wasnt a keyframe but it seems to need that. perhaps there's a clear() somewhere that is resetting things.
i wanted to reduce mesh upload bandwidth - not a big deal but should be possible to get a tiny performance improvement.

Originally posted by @capnramses in #11 (comment)

[capnramses]

• try changing the clear() so we can avoid updating indices and textures on non-keyframes and reduce redundant mesh data updates.