Do nothing in wxMemoryDC::SelectObject() if the bitmap is already selected.

It doesn't make sense to make a copy of the bitmap in order to select it into
wxMemoryDC if it's already selected into it.

See #11640.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65913 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/dcmemory.h

@@ -81,13 +81,17 @@ public:
         should use when you select a bitmap because you want to modify it, e.g.
         drawing on this DC.
 
-        Using SelectObjectAsSource() when modifying the bitmap may incurr some
+        Using SelectObjectAsSource() when modifying the bitmap may incur some
         problems related to wxBitmap being a reference counted object (see
         @ref overview_refcount).
 
-        Also, before using the updated bitmap data, make sure to select it out
-        of context first (for example by selecting ::wxNullBitmap into the device
-        context).
+        Before using the updated bitmap data, make sure to select it out of
+        context first either by selecting ::wxNullBitmap into the device
+        context or destroying the device context entirely.
+
+        If the bitmap is already selected in this device context, nothing is
+        done. If it is selected in another context, the function asserts and
+        drawing on the bitmap won't work correctly.
 
         @see wxDC::DrawBitmap()
     */

src/common/dcbase.cpp

@@ -236,6 +236,12 @@ wxMemoryDC::wxMemoryDC(wxDC *dc)
 
 void wxMemoryDC::SelectObject(wxBitmap& bmp)
 {
+    if ( bmp.IsSameAs(GetSelectedBitmap()) )
+    {
+        // Nothing to do, this bitmap is already selected.
+        return;
+    }
+
     // make sure that the given wxBitmap is not sharing its data with other
     // wxBitmap instances as its contents will be modified by any drawing
     // operation done on this DC