Fix conpty cursor movement detection on double-width lines

[j4james]

When the VT render engine checks whether the cursor has moved in the
InvalidateCursor method, it does so by comparing the origin of the
given cursor region with the last text output coordinates. But these two
values are actually from different coordinate systems, and when on a
double-width line, the x text coordinate is half of the corresponding
screen coordinate. As a result, the movement detection is sometimes
incorrect.

This PR fixes the issue by adding another field to track the last cursor
origin in screen coordinates, so we have a meaningful value to compare
against.

References and Relevant Issues

The previous cursor movement detection was added in PR #17194 to fix
issue #17117.

Validation Steps Performed

I've confirmed that the test case from issue #17232 is now fixed, and
the test case from issue #17117 is still working as expected.

PR Checklist

• Closes Cursor movement is not always detected on double-width lines #17232

[j4james]

Given the number of times I've got this wrong already, I wouldn't be surprised if there is still some edge case that I'm missing, but I think this should be correct now.

[DHowett]

Hey, bugs is bugs. Thanks for being on top of these, James!