Allow building with SDL 2 and create RenderDeviceList

Closes #1064

Building with SDL 2 is on by default. I think this is a sane choice, since SDL 2 is stable now and our primary target is desktop systems on Windows, Mac OSX, and Linux. To disable it for legacy systems, use this flag with cmake:
```
cmake -DUSE_SDL2=Off
```

The RenderDeviceList let's us pick which RenderDevice child class to use when launching. We only have one here in this repo, but it provides an easier way to [add more](https://github.com/dorkster/flare-engine-next/blob/testing/src/RenderDeviceList.cpp#L34). To specify a render device, use the `renderer` flag like so:
```
flare --game=flare-game --renderer=sdl
```
The above line is redundant, as `sdl` is the default render device to use when there's no `renderer` flag.