When using `UNPACKED_MPQS`, avoid all the SDL machinery for reading
files.
This is beneficial not only due to reduced indirection but also because
we can test for the file's existence and get the file size without
opening it, which is much faster.
libmpq is a much simpler alternative to StormLib for reading MPQ archives.
We use our own fork of libmpq: https://github.com/diasurgical/libmpq
Impact:
* DevilutionX is now a lot more portable. Unlike StormLib, libmpq only
needs platform-specific code for Windows.
* Locks around file access **removed** (instead we duplicate the file descriptor for streamed audio only).
* RAM usage is **300 KiB** lower than StormLib.
* Stripped release linux_x86_64 binary is **32 KiB** smaller.
* Amiga build now hangs instead of crashing.
Gleb Mazovetskiy
Anders Jenbo
staphen