Dynamically Check for Gray PhoneBooth

What: Currently, we perform the check
(get_node("Sprite").get_texture().get_path() ==
"res://Teleporter/GrayPhoneBooth/PhoneBoothGray.png")
in is_gray_phone_booth() in PhoneBooth.gd.

However, this is prone to break if the path to the
.png file changes and we should find a way to
dynamically check for a gray phone booth.