[hlsdl] GL.deleteProgram is missing.

In [GL.hx](https://github.com/HaxeFoundation/hashlink/blob/master/libs/sdl/sdl/GL.hx), there is no declaration for `deleteProgram` unlike the other GL object destructors such as `deleteShader`, `deleteTexture` and so on.

By the way, can I ask whether setting GL objects to null are enough to call their corresponding GL destructors? I noticed that the API looks a bit like WebGL so I wonder if garbage collected GL objects are automatically deleted.

Thank you.