MEGA65: HDMI output is not working correctly on all monitors

[sy2002]

On my monitor HDMI on MEGA65 works fine.

On Volkers it does not, he writes:

Btw. I received a Mega65 prototype and did a short test with your qnice core. It seemed to work ok, but the HDMI output did cut off quite a bit of the image on my tv.

This status quo was the VGA release V1.6. In the meantime, Michael completely redid the VGA. Our working hypothesis was, that we will not need the sync tool hdmi_de any more and that everything will just work fine everywhere.

This issue is about testing this (and fixing if necessary) and it is blocked by issue #143, which needs to be done first.

[sy2002]

Volkers feedback after running V1.6's hdmi_de tool:

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The picture is cut off about one character all around I would say. When
using the hdmi_de program, I can get a good picture by changing the
first value to <=5, the second to >=654 or the third to >=486. However,
that seems to work only because the input signal is now not recognized
as 480p anymore but as e.g. 644x480. So I assume the TV's processing
changes.

This was my current TV (LG OLED), but I also tried my old one (Samsung
Plasma). With this TV, the cut-off was a bit different and changing
values with hdmi_de did not help. I could only change the values
slightly before it did not accept the signal anymore. This TV was always
rather picky with its input signals.

Overscan was disabled on both TVs and the signal with the Mega65 core
seemed ok. But then it has a pretty huge border, so I would probably not
notice similar problems.