Fix drawing wxBitmap with mask

Under wxGTK+2, when wxBitmap has a wxMask but this mask shouldn't be used
in the actual drawing (useMask parameter in wxDC::DrawBitmap() is set to
false), we need to get bitmap data from the buffer with raw (original,
non-masked) data and not from the buffer containing data with mask applied.

Close #18498.

src/gtk/dcclient.cpp

@@ -1155,7 +1155,7 @@ void wxWindowDCImpl::DoDrawBitmap( const wxBitmap &bitmap,
         }
     }
     else if (hasAlpha || pixmap == NULL)
-        pixbuf = bitmap.GetPixbuf();
+        pixbuf = useMask ? bitmap.GetPixbuf() : bitmap.GetPixbufNoMask();
 
     if (isScaled)
     {