Fix crash when shader validation is enabled.

[squidbus]

I didn't get any feedback on #2416 whether this would be an acceptable fix, so I'm opening it as a PR in case it's wanted.

Basically it seems that for some reason when Metal shader validation is enabled, there may be a crash due to auto-release pool double-release on an MTLComputePipelineDescriptorInternal, but only when using newComputePipelineStateWithFunction. I'm not sure why this is happening or whether it's an internal Metal bug with the validation layer or MoltenVK's fault, but changing it to create a temporary MTLComputePipelineDescriptor and call newComputePipelineStateWithDescriptor resolves the issue for me.

[cdavis5e]

My only concern is that this wouldn't work on iOS 8, but I suspect there aren't many people who are going to care about that.

[squidbus]

My only concern is that this wouldn't work on iOS 8, but I suspect there aren't many people who are going to care about that.

If you're referring to the minimum version for newComputePipelineStateWithDescriptor, it is used for other pipelines already so there's no difference for compatibility using it here as well.