CPU support for dense reconstruction

Is your feature request related to a problem? Please describe. Dense reconstruction requires CUDA which is not available to machines with non-NVIDIA GPUs and Macs. As a Mac user, this means there is no way to do dense reconstruction.

Describe the solution you'd like Enable dense reconstruction to run on the CPU to make it available to MacOS and non-nvidia GPUs.

Describe alternatives you've considered I do not see any. Getting a Windows/Linux machine with a NVIDIA GPU seems the only way right now to do dense reconstruction in COLMAP.

Additional context An alternative way to solve the problem at least for Mac users would be to enable Metal GPU support as per this feature request.