Support MVK_ALLOW_METAL_EVENTS on NVIDIA and Rosetta2

[billhollings]

MoltenVK disables the use of MTLEvents for semaphores on NVIDIA and Rosetta2, since testing indicates that MTLEvents were problematic in this situation.

However, further testing indicates that this is not always the case, and we should create a runtime environment that permits MTLEvents to be used in these situations, if selected by the app.

Potentially override MVK_ALLOW_METAL_EVENTS and MVK_ALLOW_METAL_FENCES with an enum config that would allow options like "use MTLEvents always" and "prefer MTLEvents in most cases", as well as "prefer MTLFences", to allow taking into consideration the NVIDIA and Rosetta2 issues, but also allow them to be overridden by the app.

[TellowKrinkle]

BTW do we have a small, consistent reproduction of the MTLEvent lockup on Nvidia/Rosetta2? Or is it just whole games at the moment?

[billhollings]

Merge PR #1709 permits this, providing the ability to use enumerated env var MVK_CONFIG_VK_SEMAPHORE_SUPPORT_STYLE to force MTLEvents to always be used, regardless of the GPU or platform, for apps that can safely run on those platforms.