Don't use bitmap still selected in wxMemoryDC in image sample.

The bitmap must be deselected from wxMemoryDC before being used in any other
way but the sample didn't do this. Fix this by simply destroying the DC as
soon as we don't need it, this makes bitmap available for other use as well.

Closes #12310.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65232 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/image/canvas.cpp

@@ -641,9 +641,8 @@ void MyCanvas::CreateAntiAliasedBitmap()
 {
     wxBitmap bitmap( 300, 300 );
 
-    wxMemoryDC dc;
-
-    dc.SelectObject( bitmap );
+    {
+        wxMemoryDC dc(bitmap);
 
         dc.Clear();
 
@@ -655,6 +654,7 @@ void MyCanvas::CreateAntiAliasedBitmap()
         dc.SetBrush( *wxRED_BRUSH );
         dc.SetPen( *wxTRANSPARENT_PEN );
         dc.DrawRoundedRectangle( 20, 85, 200, 180, 20 );
+    }
 
     wxImage original = bitmap.ConvertToImage();
     wxImage anti( 150, 150 );