D loader, some issues
Had to change a few things in the D generated loader to get it working for me:

https://github.com/Dav1dde/glad/blob/d/glad/glad/wgl/types.d#L16
had to change to 

```
public import core.sys.windows.windows;
```

https://github.com/Dav1dde/glad/blob/d/glad/glad/wgl/loader.d#L18
had to replace with:

```
import glad.gl.loader : gladGetProcAddressPtr;
```

i.e. just use the one in gl. Without this, I get 'Previous definition different' errors. 

The public import of core.sys.windows.windows made this:
https://github.com/Dav1dde/glad/blob/d/glad/glad/wgl/loader.d#L165
conflict with another definition, so I had to comment this out. These functions are in gdi32.lib anyway, not sure why glad is trying to load them from opengl?

And finally I commented out the close_gl() lines from both wgl.loader and gl.loader. Could closing be made optional?

I am using the D loader as a derelict replacement, creating opengl context from scratch. Maybe my use case is not what is intended, and lead to these problems, not sure, just thought I'd share.

Thanks!  
