Underflow causing panic when generating mesh.

[Larbino1]

Hi there! Thanks for making this crate. I'm just getting started using it and encountering this panic when generating a mesh:

thread 'Async Compute Task Pool (0)' panicked at /home/daniel/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rerecast-0.1.1/src/contours.rs:32:21:
attempt to subtract with overflow
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Encountered a panic in system `bevy_rerecast_core::generator::poll_tasks`!
Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!

The problem seems to be that self.height - self.border_size*2 is less than zero.

This must be a silly edge case due to my scene being empty apart from a flat plane, but maybe it could be handled in some way. I'm not familiar enough with the area/package to suggest how. In my case adding another plane offset by 5.0 in each axis fixed the issue.

[janhohenheim]

Could you upload your scene? Or provide code to recreate it? :)