Starting to get a hold of the Tile struct

Iterators are turning my brain to mush. I'm trying far too hard to
leverage existing iterator tooling. The closures all over the place make
saying the type names basically impossible. For chaining in the middle
of a function, this is fine. But I need to stick it in a `struct Tile{}`
and move it between threads.

This commit is a save point for my own sanity, more than anything else.
The tile struct exists and compiles. The only changed part is the
extraction of the pixel sampling loop into a named function (instead of
an nameless closuuuruurreeee)

Cargo.toml

@@ -7,4 +7,4 @@ edition = "2021"
 
 [dependencies]
 rand = { version = "0.8.5", features = ["small_rng"] }
-
+itertools = { version = "0.11.0" }