Make the default max RAM cache size more conservative

[RyanJDick]

Summary

This PR makes the default max RAM cache size slightly more conservative. This should help with some reports of Invoke using more RAM/VRAM than expected/desired. Of course, users always have the option to configure Invoke to use a larger cache size.

QA Instructions

The change is relatively straightforward. I tested the following cases:

• Max RAM limit is set to 50% of available system RAM.
• RAM limit is set to 50% of available system RAM and then reduced to 1x VRAM cache size

Reviewers are encouraged to try this out on their systems to see if default performance is worse/better/the same.

The real test will be whether this reduces the number of issue reports with the default configs.

Merge Plan

No special instructions

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[hipsterusername]

Looks like this might not be kosher on mac, based on the tests

[RyanJDick]

Looks like this might not be kosher on mac, based on the tests

That's a known-flaky MPS test. It passed after re-running.