Add view matrix gradients & optimize cameras

[ArthurBrussee]

To enable training from rougher poses (eg. ARCore poses or other SLAM systems), it would be great to have caamera gradients.

This would mainly be a matter of patching up project_backwards to output view matrix gradients that match the ones in gSplat, some of this code is in place.

It does come with other complications:

• Adding in more bindings might push things over the binding limit on Chrome (only 10 buffers...)
• Keeping cameras as differentiable tensors will require changing the code in quite a few places. Not 100% sure how to keep that clean.

This would also help for future work in #18