Fix Vertical flip bit not working on HW and adding feature -ve y_offset

[RetroCogs]

This is to address issue #796

This is using bit 4 of screen ram byte 1 when GOTOX is set to specify that the y_offset should be subtracted from the glyph_data_address.

This also fixes the issue that when vertical flip bit was set, y_offset would be ignored.

Tested FirstShot, all RetroCogs tutorials, Unicone, Showdown, IntroDisk #1 RRB Demo, Xanadu demo

[lydon42]

Looks good to me.
Is there some piece of testcode that could be used for automated testing?
So a RRB char that is Y offset in plus and then in minus direction, each time pixels are read with the debug crosshairs to see if the change was as expected.

[RetroCogs]

So yes, when nve_offset bit is set the offset is subracted rather than the default of added.

This also fixes the fact that when vertical flip bit was set it would ignore the y_offset.

I'm happy to add some tests similar to test_671.c as a separate pull request into mega65_tools.