X11: Calculate ime cursor position correctly.

[xorgy]

Fixes #3965

[kchibisov]

This is generally not correct, though, since the intended way to do is to pass an area that shouldn't be obscured rather than offset. The concepts are different here, I'd say.

Probably something from XNPreeditArea is how it should work.

I generally won't block this patch, but it should be mentioned in the docs what it does and probably we should suggest to pass just a line generally for cursor to not hit bugs in various setups.

[xorgy]

That's fair, I can add documentation changes to that effect, and a couple comments in the backend mentioning other possible ways to do preedit area reports (though I haven't found any IM servers implementing PreeditArea so far).