[Coco] VDG Detection

[Alex Evans]

In the ROM Diagnostics thread, there is the question of whether or not it
is possible to detect through software whether an MC6847 or an MC6847T1 was
present. An idea was presented that there may be a difference in the
bumper of lines per frame. I came up with some code to test this
possibility, but I only have access to a machine with an MC6847. I have
managed to get a null result using xroar (in both cases 262 for NTSC and
312 for PAL). If there's anyone is willing to post results from actual
hardware to confirm this null result is accurate, it would be greatly
appreciated.

Attached you will find the source code for the test as well as DECB and
DragonDOS binaries.

Thank you.
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