Consider adding simulation shaders

Simulation would just be fragment shaders that read from and render to an additional texture. This texture would be accessible in all downstream shaders, and possibly downloaded to the CPU on every frame.

The issue does not name any files, tests, or entry points. Start by locating the rendering and shader pipeline in the Rust framework, then determine the scope and acceptance criteria for simulation textures and any CPU readback; the issue currently does not define when the work is done.

Written by the indexing model from the issue text.