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;

src/Spectre.Console/Internal/Extensions/StringExtensions.cs

@@ -16,6 +16,11 @@ namespace Spectre.Console.Internal
 
         public static int CellLength(this string text, RenderContext context)
         {
+            if (context is null)
+            {
+                throw new ArgumentNullException(nameof(context));
+            }
+
             return Cell.GetCellLength(context, text);
         }
 

src/Spectre.Console/Internal/Text/Cell.cs

@@ -21,6 +21,17 @@ namespace Spectre.Console.Internal
                     }
                 }
 
+                // TODO: We need to figure out why Segment.SplitLines fails
+                // if we let wcwidth (which returns -1 instead of 1)
+                // calculate the size for new line characters.
+                // That is correct from a Unicode perspective, but the
+                // algorithm was written before wcwidth was added and used
+                // to work with string length and not cell length.
+                if (rune == '\n')
+                {
+                    return 1;
+                }
+
                 return UnicodeCalculator.GetWidth(rune);
             });
         }