Fix rounding error when getting VisualLine

[trijnsburger]

A rounding error occasionally occurs here (found when using RectangleSelect), causing this method to unexpectedly return null, which in turn causes RectangleSelection.ReplaceSelectionWithText() to incorrectly set textArea.Caret.Position to (Line = 1, Column = 1).

Note that the epsilon value of 1e-12 is an estimation based on the accuracy of the visualTop value, which has 13 decimals when its integer value is in the thousands (4 digits).

[siegfriedpammer]

Can you please provide reproduction steps (using ICSharpCode.AvalonEdit.Sample.exe) for this problem? The GetVisualLineFromVisualTop method is used in a lot of contexts and I would prefer to not introduce any weird behavior due to magic constants.

I would expect the "lower than" relation to be not affected by epsilon errors.

Thanks!