decklink: reset calculation of time_mapping to fix clipping HDMI video
reset calculation of gst_decklink_video_src_update_time_mapping on no_signal. When the HDMI cable was disconnected for a long time, the calculation took too much time. SDI cable works fine.
Reset on no_signal is a simple solution. Perhaps it would be better to calculate the maximum possible latency and reset when exceeded...
Fixes #1484 (closed)