Add a simple example

[frewsxcv]

cargo run --example simple

[frewsxcv]

Though when I run it, here's the error I get:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `36054`,
 right: `36053`', /Users/coreyf/.cargo/git/checkouts/pathfinder-aa48411030fb9d2c/45daf9e/gl/src/lib.rs:472:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace