1. Moves more assets-related stuff from `init` to `engine/assets`.
2. Removes `SDL_audiolib` dependency from `soundsample.h`.
3. Cleans up some unused/missing includes.
Implements a `require` function that supports built-in modules like so:
```lua
local log = require('devilutionx.log')
```
It falls back to reading from assets, so this loads `lua/user.lua`:
```lua
local user = require('lua.user')
```
The bytecode for the asset scripts is cached, in case we want to later
support multiple isolated environments.
There may be a simpler or better way to do this.
It's good enough for now until someone more knowledgeable
about Lua comes along.
For `MultiFileLoader` and `LoadMultipleCl2Sheet`, we now open one file
at a time.
This can help on platforms that limit the number of concurrently open
files, such as the PS2.
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.
Anders Jenbo
Gleb Mazovetskiy
Eric Robinson
obligaron
staphen