mmal/vout: Configure latency target

mmal/vout: Configure latency target

The mmal video_render element allows to configure a latency target. Although
the parameter is named AUDIO_LATENCY_TARGET it is not actually related to
audio but synchronizes frame draws with hdmi vsync. It tries to match the hdmi
vsync rate to the picture render rate by slightly varying the hdmi pixel
clock. This makes only sense if the actual display refresh rate matches the
video frame rate, so that only minimal drift compensation is required. Thus
enable it only when mmal-adjust-refreshrate is activated.

Signed-off-by: Julian Scheel <julian@jusst.de>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>