* 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
* Implement audio backend configuration option
* Use OpenAL by default
* Increment version number in config.json
and add 30px to the height of the settings window
* nits
* capitalise audio backend names
