Fix rgba export / filter output based on occupancy grid

[ookey]

The Testbed::get_rgba_on_grid method enables to export rgba samples along a dense grid of the scene.
Only nerf_network::inference is used, thus keeping some points set to 0 in density grid.

This is as problem when the two info are different, eg if the latents in the empty zone have evolved due to collisions in the hashtables, or if the VR cleaning facility has been used.

This patch aims at setting to 0 all samples whose position is according to the density grid.

[Tom94]

Thanks for the contribution! Looks good to me -- and apologies for the delay.