Use official metal sources

[davidgiga1993]

The metal sources currently used by the robovm-metalangle backend are coming from a dead repo, see readme.

Libgdx should - at some point - use the official sources as they now cover a wider compatibility as the fork currently used.
The only issue is that there are no release binaries as far as I can see, so those would need to be built first, but the workflow files for this are in the fork repo, so shouldn't be too much effort.

[obigu]

While I agree moving to official Angle would be good I think it's not as simple as that. MGLKit API is not part of official Angle so the adapter framework would need to be created. kakashidinho/metalangle#81 discusses this possibility but it seems the author doesn't have the time to work on it which suggests it's not such a low effort task.