Support for variable capture resolution

[trickeydan]

Either as an Enum of resolutions, or a tuple.

Probably should be an argument to process_frame

[RealOrangeOne]

I don't think an enum of resolutions is viable, as that'll depend on what the camera supports.

We should probably also validate this before capture. It looks like OpenCV will not set an invalid or unsupported resolution, so using this to check it's set correctly is probably a good idea! https://stackoverflow.com/questions/19448078/python-opencv-access-webcam-maximum-resolution