Wide character detection might be broken with WriteConsoleOutputA

[rprichard]

From my comment, https://github.com/rprichard/winpty/issues/105#issuecomment-304430216:

The 0xFFFF invalid character in patterns 4 and 8 is weird; I'm guessing it's there for backwards compatibility? winpty doesn't currently handle it, but I guess it should. It currently expects the trailing and leading wide char to equal.

I should double-check, but I think this is a bug in winpty.