Fix not rendering textures on release builds
Fix not rendering textures on release builds

SDLDemo was not able to render the texture in release builds.

I saw it caused by `SDLRenderer.copy` then I made sure to copy a texture on the release build correctly.

Reproduction
------------

Run the demo in release build.

```bash
$ swift run -c release
```

Cause
------------

It seems to occur by optimization about `UnsafeMutablePointer`, but I can't say for certain...

PureSwift/SDL,PureSwift/SDL