Integrate Unreal Engine mtlpp enhancements

[oscarbg]

Hi, seeing commits in Unreal master branch I see:

Initial extensions to mtlpp:
- Fixed over retention of alloc-init'd objects.
- Added 10_13 & 11_0 availablity macros.
- Started, but have not yet finished adding new Metal API function wrappers.

so just saying if these will be avaiable in your repo as could be brought from Unreal mtlpp source code..

[naleksiev]

Thanks for sharing this, I wasn't aware. I would have loved a pull request. I have been making Metal 2.0 changes myself.