panic in piston example

[tpltnt]

Hi there,

the piston example panics when executed. thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: InvalidFps([0.0, 0.0])', src/libcore/result.rs:997:5 note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace. The offending line seems to be imgthread.join().unwrap();. Note that #17 is related but does not fix this panic.

Cheers, tpltnt