Fix selection bugs when terminal has yDisp > 0

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1733627

XtermSharp/SelectionService.cs

@@ -130,7 +130,7 @@ namespace XtermSharp {
 		/// </summary>
 		public void SelectAll()
 		{
-			Start = new Point (0, 0);
+			Start = new Point (0, terminal.Buffer.YDisp);
 			End = new Point (terminal.Cols - 1, terminal.Buffer.Lines.MaxLength - 1);
 
 			// set the field to bypass sending this event twice
@@ -243,7 +243,7 @@ namespace XtermSharp {
 				break;
 			}
 
-			if (start.Y < 0 || start.Y > terminal.Buffer.Lines.Length) {
+			if (start.Y < 0 || start.Y > terminal.Buffer.Lines.Length + terminal.Buffer.YDisp) {
 				return Array.Empty<Line> ();
 			}