Add a color coordinate to the chaos game

Also refactors the gasket example to avoid using shader entry points; the entry point signatures are not stable, and I don't want to keep refactoring the gasket while adding more advanced color/image features.

enkou-shaders/src/lib.rs

@@ -1,6 +1,6 @@
 //! # Enkou
 #![no_std]
-#![deny(missing_docs)]
+#![warn(missing_docs)]
 #![allow(clippy::needless_range_loop)] // SPIR-V backend has issues with iteration over items
 
 pub mod camera;