Fix scroll rendering artifacts by removing ScrollDC

ScrollDC shifted buffer pixels during parsing before dirty rows were
rendered, causing unrendered garbage to propagate into non-dirty rows
on successive scrolls. Replace with FAllDirty to repaint all rows from
cell data after any scroll. With batched TextOut this costs ~250 GDI
calls per scroll instead of the old 12,000, so the overhead is minimal.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

delphi/KPANSI.PAS

@@ -101,7 +101,6 @@ type
     procedure RecalcCellSize;
     procedure RedrawBuffers;
     procedure ResizeScreen;
-    procedure ScrollBufs(N: Integer);
     procedure SetCols(Value: Integer);
     procedure SetCursorVisible(Value: Boolean);
     procedure SetRows(Value: Integer);
@@ -761,24 +760,6 @@ begin
 end;
 
 
-procedure TKPAnsi.ScrollBufs(N: Integer);
-var
-  R:       TRect;
-  I:       Integer;
-  ScrollY: Integer;
-begin
-  if FBufDC[0] = 0 then
-    Exit;
-  ScrollY := N * FCellHeight;
-  R.Left   := 0;
-  R.Top    := 0;
-  R.Right  := FBufW;
-  R.Bottom := FBufH;
-  for I := 0 to 1 do
-    ScrollDC(FBufDC[I], 0, -ScrollY, R, R, 0, nil);
-end;
-
-
 procedure TKPAnsi.DeleteChars(N: Integer);
 var
   Line: PTermLine;
@@ -864,8 +845,7 @@ begin
   GetMem(Line, SizeOf(TTermLineRec));
   AllocLine(Line);
   FScreen.Insert(0, Line);
-  ScrollBufs(-1);
+  FAllDirty := True;
-  FDirtyRow[0] := True;
 end;
 
 
@@ -885,8 +865,7 @@ begin
   AllocLine(Line);
   FScreen.Add(Line);
   UpdateScrollbar;
-  ScrollBufs(1);
+  FAllDirty := True;
-  FDirtyRow[FRows - 1] := True;
 end;