aabb crop for rendering a cropped nerf

[vincepapaix]

Implement a similar function as nvidia instant-ngp crop aabb as seen in Jonathan's video at 6min20 https://www.youtube.com/watch?v=k67qHXRhdY4

Preference will be to have that for all nerf models, but ideally nerfacto.

=> this will be beneficial to crop out unwanted 'floaties in the FG of the video => bonus, if we can export the nerf with a matte (alpha), to be composited on other videos.

thanks