show/showAll methods now use Canvas instead of SDL_Surface

- added compatibility method to Canvas to allow SDL_Surface access
- added drawBorder method to Canvas to replace CSDL_Ext method
- added drawColor method to Canvas to replace CSDL_Ext method
- minor changes to Tavern and Trade windows to adapt to new API

client/mainmenu/CreditsScreen.cpp

@@ -34,7 +34,7 @@ CreditsScreen::CreditsScreen(Rect rect)
 	credits->scrollTextTo(-600); // move all text below the screen
 }
 
-void CreditsScreen::show(SDL_Surface * to)
+void CreditsScreen::show(Canvas & to)
 {
 	CIntObject::show(to);
 	positionCounter++;