Allow resolution to be configured on command line. Default to 1280x1024.
Change default back to 800x600 as mac travis builders don't work with 1280x1024
Change opts to work in screen pixels that are scaled to device pixels.
Merge pull request #3398 from glennw/config-res

Allow resolution to be configured on command line. Default to 1280x1024.

Reviewed-by: SimonSapin