- cleanup on the tile management.

* removed the long defunct EDuke32-based map art system. With our move to named textures this will not work anymore so fixing the existing problems would be pointless.
* moving .def related parsing stuff out of buildtiles.cpp.
* some initial refactoring to make the tile store a static, non-modifyable resource after initial setup.

source/core/mainloop.cpp

@@ -347,7 +347,6 @@ static void GameTicker()
 	{
 	default:
 	case GS_STARTUP:
-		artClearMapArt();
 		gi->Startup();
 		break;