Backport SDL 2-dependent new input/sound/platform features to Windows 98

[nmlgc]

SDL 2 doesn't work on Windows 98 by default, which means that the i586 builds are going to miss out on the following features:

• Expanded game controller support introduced by Upgrade from DirectInput 7 to a newer input API (would mainly help with antivirus false-positives) #22
• Add SC-88Pro recordings as an alternative to the MIDI soundtrack #9
• Support keyboard remapping #48
• Support joypad remapping of directional inputs #51
• Store separate bindings for every different controller #52
• Opening the SC-88Pro BGM pack download page in the system's default browser from inside the game.

Making these work on Windows 98 could be done in two ways:

1. Get SDL 2 running on Windows 98 after all, forking it if necessary. More expensive, but would benefit every SDL 2 game. Bonus points for getting SDL 2 to run without KernelEx.
2. Backport all features to the original DirectInput and DirectSound backends. Cheaper, but will only cover Shuusou Gyoku.

It's up to the backers to decide which strategy I go with.

[nmlgc]

Turns out that miniaudio has two disadvantages when it comes to the features needed for #9:

1. It doesn't provide APIs for accessing FLAC or Ogg metadata, even though its vendored libraries have the code for it.
2. Its fader is linear rather than logarithmic, with no way to customize it.

With 1) forcing me to separately include the same libraries anyway, I've moved FLAC/Vorbis decoding and fading to the platform-independent game layer, using an adapted version of thcrap's BGM streaming code. miniaudio now only receives the raw audio stream, which means that we only ended up using it for features that DirectSound also supports.

Thus, it would make slightly more sense to go for 2) in the case of sound. Then, we not only get FLAC/Vorbis BGM modding on Windows 98, but even the modern SDL Windows build could ditch miniaudio and go back to DirectSound, reducing binary size and release build link times.

[nmlgc]

Option 1 (porting SDL 2 to Windows 98) has received €62.50 in microtransaction funding from M0003.