add entry point for dfhooks_sdl_loop

[myk002]

FYI: @dhthwy

Note that DF doesn't actually call this hook yet, and won't until at least 50.16/51.01-beta27

Until then, you can probably synthesize this call by tapping into dfhooks_sdl_event and rate limiting with a timer

[ab9rf]

we can get a better approximation by interposing renderer_2d::render, which is called from nearly the same point that the render hook is expected to be inserted

[dhthwy]

cool. Yeah, renderer_2d::render is already being used, and rate limiting is in place. Should be easy to switch it.

20 fps normal and 10 fps while minimized (for preview rendering), no render at all if window isn't shown (except for minimized). iirc.

Reminds me, I need to push some updates.