a0a93e0cdb
Some clients always set blend state for all eight attachments, even if the attachments are unused. Since the attachments are unused, the expectation is that the corresponding blend states won't matter. But in Metal, there's no way to tell if an attachment won't be used. So, if you enable blending on an unused attachment, its pixel format will be `MTLPixelFormatInvalid`, which doesn't support blending; Metal will assert on this.