This removes the hack that screenshot_save() got the colorspace
information from the decoder. Instead, require the VOs to set the
colorspace information on the mp_images used to pass around the
screenshot data. This is more correct, as the image may have been
converted/modified in the video filter chain, although there's
nothing yet in the video filter chain which does this correctly.
Although slightly less precise, this sounds less clunky.
This change also causes the --screenshot-filetype option to be renamed
to --screenshot-format.
Most of these are useless or probably even dangerous. Support them
anyway, because it's easy, and we want to replace vo_jpeg without any
disadvantages.
