Yeah, the trailer is a real game video (what I spent a week on earlier when I didn't have a dev log.)
I'm building on the old code currently - and I haven't found a reason to change the basic structure there yet (certainly there'll be extra grids etc. in time.) Every surface there was already allocated separately; the atlas is long gone apparently, and there isn't any large-scale swapping or binding. It just blits separate little surfaces from a non-texture-atlas catalog by index and coloration (as the number of textures grows, pulling from that catalog might also need to be checked? Not sure what experiences people have had there if any.) It only refreshes tiles that are updated (though of course when you scroll, most tiles are changed, so you can't 100% rely on that - so it's important to make sure there aren't tons of layers used everywhere), and that carries over to multiple layers -- I just have a few extra flags to check on the refresh. There's *potentially* an optimization problem there, doing a few extra checks on the screen texture index arrays, but so far so good (that is, 100 FPS on my crappy computer, and I also get 100 FPS when I just draw every tile.)
My plan is to release all of my code changes, just as I've been doing up to this point. The free Dwarf Fortress Classic Linux version will have the same folder as usual, with all the same files (and any new ones), and that'll be the same (basically) as the Steam code. My understanding is that the licenses from the contributors are in order - at the time, I requested BSD/MIT or equivalent/looser licenses, as I wanted anybody to be able to use the code in their projects, no matter if they were commercial or not, as a condition of using the code myself.
So yeah, I'm going to be sharing all of the new rendering stuff, in basically the same format as before, and between classic and steam it shouldn't be all that different. That's the plan anyway! Certainly if there are issues, hashing them out now rather than later is good.