Screen: Add pixeldepth 5 "YUV" support to libdc1394 engine.

So usercode can explicitely request YUV capture from a camera
to save bus bandwidth on the firewire bus.
Screen: Make framerate calculation in firewire mode 7 less wrong.

Don't have a camera that doesn't report bogus values, so this
is probably still wrong and only the fallback path reports
correct values. However, the old formula was definitely wrong,
so this is less wrong.