graphics: begin to rewrite code to be more simple: - add

a struct Texture to store a sf::Texture and the application's texture sizes,
 - add a texture resources manager and retrieve codes to allow - if needed,
 rescaling startsplash and spidlogo at startup - loading some graphics with
 different resolutions:     buttons, inventory, pcedbuttons, statarea,
 startanim,     startbut, startup, terscreen, textbar, transcript   can be
 used to load high resolution pictures.

Note: currently, render_image and res_image are a mess as Texture and sf:Texture classes
  cohabit :-~

src/scenedit/scen.core.cpp

@@ -3355,7 +3355,7 @@ void edit_custom_sheets() {
 		spec_scen_g.init_sheet(0);
 		spec_scen_g.sheets[0]->copyToImage().saveToFile((pic_dir/"sheet0.png").string().c_str());
 		all_pics.insert(all_pics.begin(), 0);
-		ResMgr::graphics.pushPath(pic_dir);
+		ResMgr::textures.pushPath(pic_dir);
 	}
 	
 	set_cursor(watch_curs);