Show real chroma decoder output in ld-analyse line scope

[atsampson]

At present, the ld-analyse line scope has its own simple filters that are used to show luma and chroma components on the scope. It could show the chroma decoder's output here instead, which would be useful for debugging the decoders.

This would be easier if the decoders provided YUV or YIQ output, so it would make sense to do this as part of #283.

[atsampson]

YUV output from the chroma decoders would also mean we could implement a vectorscope, which would be very useful for adjusting chroma gain and phase.

[atsampson]

@Gamnn suggested that we could also have a luma histogram for adjusting black/white levels along with this.