Introduce comparison tool so FFP mode can be removed

Apologies for the clunky title, but this merits being more visible on the tracker, given that this issue is blocking !4771 (merged), which is itself a blocker for the actual really nice things we've wanted to have, like clustered shading, along with all manner of bindless rendering. To resolve this issue to AnyOldName3's satisfaction, the resulting tool will need to... I'll just repost his last words about it here:

Like I've said all along, I need to be able to do something in the original engine that gives me a screenshot plus something I can repeatedly load in OpenMW to reproduce the conditions of the screenshot to hopefully make a matching one, and then keep trying again until I've changed OpenMW's rendering enough to get one. It'd be enough to have a hotkey that made the original engine make a save (so actors etc. were in the right place) and a screenshot in the same frame and write exactly what the camera transform was somewhere, and then I could manually put the save through the ESS importer, then a mod for OpenMW could make it so when the save was loaded, it made the camera transform match the saved one from the vanilla engine and took a screenshot.

The transfer needs to be from vanilla to OpenMW as the vanilla engine's rendering doesn't change, but if I'm modifying OpenMW, it'll change a bunch of times and I'll want to see if it's got more or less like the reference value, and it just adds extra steps if I need to make a new reference value each time.