Expose EmbeddedResourceLoader to c api
### Background
This PR exposes the c api to the EmbeddedResourceLoader, to prevent some of the issues mentioned [here](https://github.com/servo/pathfinder/issues/234).
### Approach
- Followed `PFFilesystemResourceLoaderLocate` and simply exposed the ERL's new and boxed it in the ResourceLoaderWrapper
- Used the `Create` terminology in the signature to match the rest of the api where a new() is called
### Notes
- I successfully tested it in a little .NET binding project I'm working on.
- Moved the \`gl\` section comment down with the GL methods, as it seemed a little out of place. added a \`resources\` one. then moved the destroy fn in to the new section.
Let me know if I misinterpreted anything or if there are any changes, stylistic or otherwise, that you'd prefer. Thanks!