SDL2 submodule broken

```
> git submodule update --depth 1
error: no such remote ref 8be6e5ca9041dfcf8b20c85e43eb3beaa6efdd2b
Fetched in submodule path 'vendor/sdl2', but it did not contain 8be6e5ca9041dfcf8b20c85e43eb3beaa6efdd2b. Direct fetching of that commit failed.
```