ATV demod fixes
The AM demod was not working properly at all. It was missing a proper AGC and therefore the video signal was almost never scaled properly. If the signal is stable enough (rapid fades are an issue in AM anyway) and the images not too dark for a long period the video signal should be scaled to the [0, 1] range appropriately after a short while. Two more controls have been added for fine tuning, Check the readme for details.
In addition the horizontal sync has been revised which should make it more robust. In particular the horizontal pulse length control is now a percentage from 1 to 100% of the nominal time as exceeding the nominal time does not work and thus is useless.
This implements issue #459