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.
https://github.com/diasurgical/devilutionx-mpq-tools produces an unpacked MPQ
with all the graphics converted to CLX and the unused files removed.
This is primarily useful on RAM-constrained platforms, such as PS2,
because it eliminates the MPQ overhead.
Adds a build option to load from such unpacked directories instead of the MPQ.
These directories are searched for in the same locations
where the MPQs would be searched for otherwise.
Example directory layout:
* /usr/local/share/diasurgical/devilutionx/diabdat/ -- unpacked and converted diabdat.mpq
* /usr/local/share/diasurgical/devilutionx/hellfire/ -- unpacked and converted hellfire MPQs (all of them merged into 1 directory)
* /usr/local/share/diasurgical/devilutionx/fonts/ -- unpacked fonts.mpq
* /usr/local/share/diasurgical/devilutionx/pl/ -- unpacked pl.mpq
These directory structure is produced by calling `unpack_and_minify_mpq`
Gleb Mazovetskiy