Reverse Engineering the SGI Indy Monitor Detection, or "thank god someone added SGI indy / indigo 2 support to MAME"

I have a bit of a soft spot in my heart for the SGI Indy and (purple, not teal, heh) Indigo 2. So imagine my surprise when NetBSD "almost" booted just fine on the Indy I have acquired. R4600PC-100, XL8 graphics .. and wonky console colours in netbsd / wonky xorg. The first deep dive is "why are the monitor colours so unpredictable?" and that got me into a fun deep dive into how the SGI Indy…