Clean up recording.h a bit.

Try to make as many things as possible private.
Pull up the size() implementation to Res_mem.
Use ResName deeper into Resnamaner and Resfile.
Mark Resmanager::get as const.
Make all Resmanager methods non-virtual.
Avoid including resmanager.h from res.h.

This avoid recompiling too much when making changes to resmanager.h.
Add a Resmanager::get that returns an std::string.
Make Res non-copyable.
Make Res more const-correct.
Inline the temporaries for loading sound samples.
Fix a linking issue with dumpwad.
Make the input of Png a const Res&.