Gqrx feature preview: IQ file quick search (final state)

1. Start IQ playback
2. Stop the DSP
3. Seek the file
Waterfall updates on
IQ file position change
FFT size change
FFT rate change
FFT window change
FFT split change
WF scale and zoom change
FFT center change
FFT colormap change
Clicking the waterfall to tune to another frequency results in both tuning and seeking the IQ file. Limitation: IQ file seek is executed only in case of actual tuning, not when the same frequency is picked.

Patched gr-osmosdr is recommended as it may crash due to memory overwrites and may produce gaps on stop/start.