Add metal framework

[aclysma]

This change was missing in my previous PR #1030. It fixes these link errors:

            "_OBJC_CLASS_$_MTLSamplerDescriptor", referenced from:
                objc-class-ref in libsdl2_sys-c107b63fb2e1f32f.rlib(SDL_render_metal.m.o)
            "_OBJC_CLASS_$_MTLTextureDescriptor", referenced from:
                objc-class-ref in libsdl2_sys-c107b63fb2e1f32f.rlib(SDL_render_metal.m.o)
            "_OBJC_CLASS_$_MTLRenderPassDescriptor", referenced from:
                objc-class-ref in libsdl2_sys-c107b63fb2e1f32f.rlib(SDL_render_metal.m.o)
            "_OBJC_CLASS_$_MTLVertexDescriptor", referenced from:
                objc-class-ref in libsdl2_sys-c107b63fb2e1f32f.rlib(SDL_render_metal.m.o)
            "_OBJC_CLASS_$_MTLRenderPipelineDescriptor", referenced from:
                objc-class-ref in libsdl2_sys-c107b63fb2e1f32f.rlib(SDL_render_metal.m.o)
            "_MTLCreateSystemDefaultDevice", referenced from:
                _METAL_CreateRenderer in libsdl2_sys-c107b63fb2e1f32f.rlib(SDL_render_metal.m.o)

I think I had this change locally applied and didn't realize it was needed in addition to #1030. I'm not sure what changed between SDL 2.0.10 and 2.0.12 that made including this framework necessary.

[aclysma]

Also, CI didn't catch this. Might be worth adding a rule to build bundled (but I also understand that it may not be practical to CI every possible permutation of features/platform.)

[Cobrand]

Thanks for the PR!

Bundled is not part of CI currently partly by choice, much like rust's tier 3 targets it's "there" but not guaranteed to always work. As you can see there are a lot of variables that we have to take into account to make it work well under all circumstances for all platforms, so the current state of bundled is "it's there, but sometimes it breaks". I don't want every third PR to be rejected because CI breaks for a platform I don't understand, just because there is an issue in CMake or whatever. I barely have the time to check new issues/PRs in time as it is...