Fix crash on resource reload

- Backend wasn't properly cleaning up its state
 - Sneaky error in WorldAttached

src/main/java/com/jozufozu/flywheel/backend/Backend.java

@@ -60,7 +60,9 @@ public class Backend {
 
 	void clearContexts() {
 		SpecMetaRegistry.clear();
+		programSpecRegistry.clear();
 		contexts.forEach(IShaderContext::delete);
+		contexts.clear();
 		materialRegistry.clear();
 	}
 

src/main/java/com/jozufozu/flywheel/backend/instancing/InstancedRenderDispatcher.java

@@ -102,8 +102,7 @@ public class InstancedRenderDispatcher {
 	}
 
 	public static void loadAllInWorld(ClientWorld world) {
-		materialManagers.get(world)
-				.delete();
+		materialManagers.replace(world, MaterialManager::delete);
 
 		InstanceManager<TileEntity> tiles = tileInstanceManager.replace(world);
 		world.loadedTileEntityList.forEach(tiles::add);

src/main/java/com/jozufozu/flywheel/util/WorldAttached.java

@@ -50,6 +50,7 @@ public class WorldAttached<T> {
 	public T replace(IWorld world, Consumer<T> finalizer) {
 		T remove = attached.remove(world);
 
+		if (remove != null)
 			finalizer.accept(remove);
 
 		return get(world);