Position mismatch between highlight marks and Xcode's error and warning marks in the text view scroller

[fortinmike]

If an error precedes an highlighted range and another follows it, Xcode's marks for those errors won't necessarily be drawn before and after our highlight mark in the scroller.

This is due to the fact that Xcode uses a different algorithm than we do to calculate the actual position to draw its error and warning marks. Further investigation would be required to match Xcode's behaviour when drawing highlight marks (if that's what we really want).