PPL support

Used for static prop lightmaps automatically generated by Src13MP's VRAD; able to be losslessly remuxed to VTF (potentially multiple, if LoDs are involved).

References:
* https://developer.valvesoftware.com/wiki/PPL
* https://github.com/ValveSoftware/source-sdk-2013/blob/master/mp/src/public/materialsystem/hardwaretexels.h