Allow creating reconstructions for which the keyframe data does not fit into the GPU memory

[puzzlepaint]

Right now, keyframes take up a lot of GPU memory, which limits the size of the reconstructions that can be made. It might be helpful to introduce the possibility to store keyframe data on the CPU while it is not used to be able to make larger reconstructions. This would likely also make the process very slow as soon as this is used, however, due to the frequent memory transfers between the CPU and GPU which will be needed then.