Base version (SDL 2.0.9) is outdated

[JayFoxRox]

We should rebase to the latest upstream.

The upstream of SDL2 is at 2.0.12 by now. However, the nxdk version is still stuck on 2.0.9.

Interesting changes for SDL2 are primarily in the input portion, which will allow querying the device per player index, and functions to query the device type (although original Xbox is not a supported style yet).