Fix rendering bug when splitting lines

The bug might occur if there are wide characters such as emojis
at the end of a line. The SplitLines method mixed cell width
and text length, which might give incorrect results. This commit
makes sure that comparison and calculation is done using cell width
where it's appropriate.

src/Spectre.Console/Internal/Aligner.cs

@@ -57,7 +57,7 @@ namespace Spectre.Console.Internal
                 return;
             }
 
-            var width = Segment.CellLength(context, segments);
+            var width = Segment.CellCount(context, segments);
             if (width >= maxWidth)
             {
                 return;