Cast for a conditional expression that won't otherwise compile on gcc3. r=bryner sr=beard b=153525

gfx/src/mac/nsImageMac.cpp

@@ -835,7 +835,7 @@ void nsImageMac::CopyBitsWithMask(const BitMap* srcBits, const BitMap* maskBits,
       ::GetClip(clipRegion);
     }
     
-    ::CopyDeepMask(srcBits, maskBits, destBits, &srcRect, &maskRect, &destRect, srcCopy, inDrawingToPort ? clipRegion : nsnull);
+    ::CopyDeepMask(srcBits, maskBits, destBits, &srcRect, &maskRect, &destRect, srcCopy, inDrawingToPort ? clipRegion : (RgnHandle)nsnull);
   }
   else
   {