* audio: Implement a SDL2 backend
This adds support to SDL2 as an audio backend.
It has the same compatibility level as OpenAL without its issues.
I also took the liberty of restructuring the SDL2 code to have one
shared project between audio and input.
The configuration version was also incremented.
* Address gdkchan's comments
* Fix update logic
* Add an heuristic to pick the correct target sample count wanted by the game
* Address gdkchan's comments
* Address Ac_k's comments
* Fix audren output
* Address gdkchan's comments
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