win-dshow: Do not allow H264 to have same priority as MJPEG
This prevents VideoFormat::Any from unintentionally selecting H264 when
MJPEG is the only other format available.
This fixes a bug where certain devices (Logitech C920 with latest
drivers) will only have H264 and MJPEG available, and using
VideoFormat::Any will then select H264 over MJPEG because it's the first
format value and has the same priority as MJPEG. So now, MJPEG will be
prioritized over H264 instead.
jp9000