Sdk and Moltenvk Regressions

[freesig]

Seems that the vulkan_sdk and moltenvk are pretty commonly introducing regressions.
#372 #332 google/shaderc-rs#58
In the interest of stability I'm recommending we setup a bit of an automated proxy / screening to test new releases.
So instead of moltenvk automatically getting the latest we could have a mirror url that points to the latest tested and stable release.
I think we might be able to set this up using circleci as they support macos.
We could download the latest release and runs some basic test to see if it crashes.
This could even go as far as checking for visual differences. I saw some game engines do this. I think it would be good practice for new nannou releases as well.
Thoughts @mitchmindtree @JoshuaBatty

[mitchmindtree]

This sounds like a great idea - it seems as though almost every second moltenvk release has caused a new issue while fixing others since we've started using vulkano.

Perhaps rather than retrieving the latest sdk automatically, we can fetch the last SDK that we've tested and confirmed works, perhaps stored in a repo under the nannou-org?