render: Manage memory for SDL_Renderer* at higher level.

Previously, each backend would allocate and free the renderer struct. Now
the higher level does it, so the backends only manage their private resources.

This removes some boilerplate and avoids some potential accidents.

src/render/SDL_sysrender.h

@@ -295,7 +295,7 @@ struct SDL_Renderer
 /* Define the SDL render driver structure */
 struct SDL_RenderDriver
 {
-    SDL_Renderer *(*CreateRenderer)(SDL_Window *window, SDL_PropertiesID props);
+    int (*CreateRenderer)(SDL_Renderer *renderer, SDL_Window *window, SDL_PropertiesID props);
 
     /* Info about the renderer capabilities */
     SDL_RendererInfo info;