[Lazarus] Mac Drawing

[DHaxton at Audiovox.com]

I have a problem getting a portion of a bitmap to reverse on a Mac. I 
initially thought about using the InvertRect API functions for mac and 
Win32, but quickly figured out that they were very different indeed. 
Here's the code I'm currently testing:

procedure TForm1.InvertRect;
var
  r: TRect;
begin
  r.TopLeft := p1;
  r.BottomRight := p2;
  Image1.Canvas.CopyMode := cmDstInvert;
  Image1.Canvas.CopyRect(r, Image1.Canvas, r);
  Image1.Canvas.CopyMode := cmSrcCopy;
end; 

p1 and p2 are points set in my mouse events - works like a charm on 
Windows, does absolutely nothing on the Mac.  I'm running 0.9.29. 
Hints, clues, or workarounds appreciated!

Thanks,
Dave H.
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