Allow output.screen window size to be configured, to deal with very large images

[leeping-ng]

For large videos (e.g. 1920x1080), the window size of output.screen could be larger than a laptop screen, and therefore part of the image cannot be seen.

One way to solve this is to change the resolution in input.live and input.recorded, but this would affect the size of the saved image/video if using output.media_writer.

A possible solution is to do a resize within output.screen itself using this: https://stackoverflow.com/questions/35180764/opencv-python-image-too-big-to-display