Serpinsky Triangle via Line Drawing

https://en.wikipedia.org/wiki/Sierpi%C5%84ski_triangle

Chaos Game:

The "chaos game" algorithm for generating the fractal is very similar to the type of process that was used for the Barnsley fern fractal #39.

Continuous Line / Koch Snowflake-Style

Might be worth implementing with some other algorithm too. The me with a continuous line was neat, and would make an awesome animation...

Simple Recursive Algorithm:

I think the simplest (and fastest) implementation would just be something like this:

• draw outer triangle boundary as lines
• start recursion:
  • draw upside-down inner triangle as lines
  • recurse into each of the three corner triangles
  • iterate until pixel distance between the bottom corners of the triangle is ~3 pixels
    • this can be precomputed into a fixed recursion depth

To implement this, we'll need line drawing. Luckily, there is a crate for that, built on the image crate that we're already using. Here is the exact method call that I think we would want to use:
https://docs.rs/imageproc/latest/imageproc/drawing/fn.draw_antialiased_line_segment_mut.html

Start by reviewing the existing Barnsley fern fractal implementation referenced as issue #39 and the project's current use of the Rust image crate. Then evaluate imageproc's draw_antialiased_line_segment_mut for the outer and recursive triangle lines. Done means rendering a Sierpiński triangle recursively until the triangle's bottom corners are about 3 pixels apart.