vf_screenshot: better check for pixel format swscale support

vf_screenshot: better check for pixel format swscale support

vf_screenshot checked for a list of pixel formats that were known to
work with swscale. However, the list was incomplete. If a pixel format
was used that was not on the vf_screenshot list, but was supported
both by swscale and the VO driver, mplayer2 would insert a useless
scale filter to convert to a format supported by vf_screenshot. Fix
this by making vf_screenshot check directly whether the pixel format
is supported by swscale with sws_isSupportedInput().