Fix trailing newlines in textboxes when using NetCore/Skia

Fixes issue where if a textbox's text ended with a newline character the additional line at the end would not render. Did this by, in these cases, adding an additional blank AvaloniaFormattedTextLine.

This also fixes an issue where line widths were being measured incorrectly because the Substring() call used for that measurement was using line.Start before it was initialized for the current line.

src/Skia/Avalonia.Skia/FormattedTextImpl.cs

@@ -560,13 +560,11 @@ namespace Avalonia.Skia
                 }
 
                 measured = LineBreak(Text, curOff, length, _paint, constraint, out trailingnumber);
-
                 AvaloniaFormattedTextLine line = new AvaloniaFormattedTextLine();
                 line.TextLength = measured;
-
+                line.Start = curOff;
                 subString = Text.Substring(line.Start, line.TextLength);
                 lineWidth = _paint.MeasureText(subString);
-                line.Start = curOff;
                 line.Length = measured - trailingnumber;
                 line.Width = lineWidth;
                 line.Height = _lineHeight;
@@ -575,10 +573,33 @@ namespace Avalonia.Skia
                 _skiaLines.Add(line);
 
                 curY += _lineHeight;
-
                 curY += mLeading;
-
                 curOff += measured;
+
+                //if this is the last line and there are trailing newline characters then
+                //insert a additional line
+                if (curOff >= length)
+                {
+                    var subStringMinusNewlines = subString.TrimEnd('\n', '\r');
+                    var lengthDiff = subString.Length - subStringMinusNewlines.Length;
+                    if (lengthDiff > 0)
+                    {
+                        AvaloniaFormattedTextLine lastLine = new AvaloniaFormattedTextLine();
+                        lastLine.TextLength = lengthDiff;
+                        lastLine.Start = curOff - lengthDiff;
+                        var lastLineSubString = Text.Substring(line.Start, line.TextLength);
+                        var lastLineWidth = _paint.MeasureText(lastLineSubString);
+                        lastLine.Length = 0;
+                        lastLine.Width = lastLineWidth;
+                        lastLine.Height = _lineHeight;
+                        lastLine.Top = curY;
+
+                        _skiaLines.Add(lastLine);
+
+                        curY += _lineHeight;
+                        curY += mLeading;
+                    }
+                }
             }
 
             // Now convert to Avalonia data formats