Fix flickering with batching

- Call syncPoint before submitting tasks
- Reset DrawBuffer even if it has no vertices when drawing

src/main/java/com/jozufozu/flywheel/backend/instancing/batching/BatchingDrawTracker.java

@@ -53,8 +53,9 @@ public class BatchingDrawTracker {
 			BufferBuilderExtension scratch = (BufferBuilderExtension) this.scratch;
 			buffer.inject(scratch);
 			renderType.end(this.scratch, 0, 0, 0);
-			buffer.reset();
 		}
+
+		buffer.reset();
 	}
 
 	/**

src/main/java/com/jozufozu/flywheel/backend/instancing/batching/BatchingEngine.java

@@ -48,6 +48,8 @@ public class BatchingEngine implements Engine {
 		var stack = FlwUtil.copyPoseStack(context.stack());
 		stack.translate(-cameraPos.x, -cameraPos.y, -cameraPos.z);
 
+		// TODO: async task engine barriers
+		taskEngine.syncPoint();
 		submitTasks(taskEngine, stack, context.level());
 	}