Portals for RP Engine

Runs on Spigot as well as Paper, and the window shows for everybody

• Portals now runs on Spigot 26.2 as well as Paper 26.2, from the same jar.
• On Paper you still keep your speed through a portal in one smooth teleport, and the far side still loads in the background. On Spigot the same thing happens the plain Bukkit way.
• Needs RP Engine 0.2.11. With 0.2.10, players' clients refused the server's resource pack, so the ring and the window never showed.
• Fixed: a portal with glass (or stained glass) in view of its far side failed to open.