Add an SDL2 example

[aclysma]

This change adds an example that uses SDL2 instead of winit.

This works great for macOS but SDL2 requires extra steps to link in windows. I usually use the bundled feature of the SDL2 crate which compiles SDL2, but it's broken in the latest version of the crate. So you might not want to take this change as it is now. (dev-dependencies can't be optional unfortunately). We could set sdl2-sys to pull in version = 0.34.2 and use the bundled feature. That would probably work well but it does add build times.

This change includes the other PRs.