[Impeller] Use NSAutoReleasePool for Metal playgrounds.

[dnfield]

Fixes flutter/flutter#117339

I judged this better than having a separate ObjC++ implementation for macOS playgrounds.

[flutter-dashboard]

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[chinmaygarde]

Oh, makes sense! Could this also explain the leak of the command buffers? The buffers hold onto resources and that could explain the massive leaks. Perhaps we can back out that commit now.

I also see a scoped NSObject for autorelease pools in fml. But the implementation is missing and the comment mentions a missing alternative. In a separate patch, we should clean that up move these utilities to FML.

Either way, lgtm. This is a massive improvement to the playgrounds experience.

[bdero]

LGTM

[dnfield]

The command buffer leak is a separate issue - applying this patch doesn't seem to help it (they're actual leaks, whereas this is just flushing the autorelease pool on each turn of the loop)