command: add property that estimates current video FPS

This is done after filters, so things like framerate-doubling
deinterlacing is accounted for.

Unfortunately, framedropping can cause inaccuracies (especially after
precise seeks), and we can't really know when that happens. Even though
we know that the decoder might drop a frame if we request it to do so,
we don't know when the dropped frame will start or stop affecting the
video filter chain. Video filters can have frames buffered, and we
can't tell at which point the dropped frame would have been output.
It's not even possible to mark a discontinuity after seek, because
again we don't know if the filter chain still has the discontinuity
within its buffers.

So we have to live with the fact that the output of this property can
be completely broken after seek, unless --no-hr-seek-framedrop is used.