Fixed errors when going through Object menu pages quickly.

One type of error that was occuring was the scene tree calling
`can_process()` on pieces that had just left the scene tree, but they
were still in the "idle_physics_process" group as the _physics_process
notification was being fired. This was solved by not removing the piece
from the scene tree early, and letting the queue_free() operation
remove it from the scene tree instead.

The game would also sometimes freeze at random intervals, with no errors
being thrown. After some debugging, I found that this freezing was
originating from the `load` call in `ResourceManager`, which was being
run inside a thread - thus I think the issue is the same as
godotengine/godot#55566. I implemented the same workaround mentioned
in that issue, and the game no longer freezes.

This commit closes #216.

game/Scripts/Game/UI/Previews/ObjectPreview.gd

@@ -221,7 +221,13 @@ func _remove_piece() -> void:
 	# case one escapes the _piece reference.
 	for child in _viewport.get_children():
 		if child is RigidBody:
-			_viewport.remove_child(child)
+			# Removing the piece from the scene tree early sometimes causes
+			# errors when the _physics_process notification fires, and it is
+			# still expecting the piece to be in the scene tree.
+			# TODO: Maybe stop removing children from the scene tree before
+			# calling queue_free() on them?
+			#_viewport.remove_child(child)
+
 			ResourceManager.queue_free_object(child)
 
 	_piece = null

game/Scripts/ResourceManager.gd

@@ -41,6 +41,11 @@ var _stex_refs = {}
 # path: The path of the resource to load.
 func load_res(path: String) -> Resource:
 	_res_mutex.lock()
+
+	# Workaround for this issue:
+	# https://github.com/godotengine/godot/issues/55566
+	OS.delay_msec(1)
+
 	var res: Resource = load(path)
 
 	# On macOS, I came across a bug in GLES3 where a StreamTexture would be