Prevent stack-use-after-scope in rendering_device_driver_metal.mm

[nikitalita]

This fixes a stack-use-after-scope error that I was getting on MacOS with sanitizers:

The problem is when it tries to get a reference from get_entry_points_and_stages().front(), but since the SmallVector that get_entry_points_and_stages() passes back is dynamically created, it deconstructs right after front() passes back that reference, thus we keep a reference to something that is no longer in scope.

Keeping that vector in scope fixes the issue.

asan.log

[nikitalita]

cc @bruvzg

[clayjohn]

Looks safe enough. If Objective-C is anything like C++ I would expect that the release builds would compile these to the same code anyway

[Repiteo]

Thanks!