Formalize internal rendering interface

[w23]

Currently there's vk_render.c which is kind of both collecting/passing though models for rendering, and doing the traditional rendering.
There's vk_rtx.c which is ray tracing rendering main core.
There's also vk_framectl.c which is being painfully aware of which renderer we're using.

This state of affairs doesn't bring joy.

What we could do instead is:

1. Split vk_render.c into:
   • "proxy", which collects model rendering requests in sequence (they are ordered by the engine/scene)
   • traditional renderer, which performs the rendering of a given sequence
2. Devise a Performer API, which suits both traditional and ray tracing renderer
3. Make both trad and rt renderer modules implementing this API.
4. Make framectl unaware of specific renderer. It should just pick the right performer from a list based on cvar. Or, maybe, the proxy could do that?
   • The only nasty quirk here is render-pass opening before or after the renderer: RT does not need renderpass for itself, but 2D overlay draws onto the RT result. Trad does use the renderpass fully. Maybe the renderer could have both function for before and after renderpass?

Ultimately, if meatpipe could be expanded enough, even trad renderer could be backed by just a meatpipe configuration, and we could have arbitrary renderers, pluggable, reloadable, and hot-swappable in realtime.