Remove SDL binaries & adapt build scripts so SDL can be built locally (without CI)

[Susko3]

The end goal for this issue is to remove SDL binaries from this repository. Instead, the native libs should be built as needed: when testing locally and when deploying to nuget.

This will make deploying to nuget a one step process (as opposed to current Build native → Deploy). Testing local SDL changes will also be easier (I currently build SDL with Rider, and then copy the DLLs).

As a bonus, we can build SDL in debug mode when testing. SDL might hit an assertion that makes debugging easier (and we know it's not our code, but SDL's that has issues).

[sr229]

I'll see if I can take a crack into this, especially if we want to work with multiarch compilations.