The first person and third person layers used hard coded IDs. If these layers
are already in use in a Unity project, this use can lead to conflicts between
the layers used by UniVRM and those inside the project. This patch allows using
Unity's layer settings to assign layer IDs through names ("VRMFirstPersonOnly"
and "VRMThirdPersonOnly").
If no named layers are found, the predefined layer IDs will be used. Since the
layer IDs are accessed only during model setup, there should be no performance
impact from this change. The names of the fields storing the layer IDs have not
been changed to keep API compatibility.
This issue was originally noticed by @Deatrathias (Virtual_Deat on Twitter).