jpg render fails with an ugly error if the file is too big

[erikmannerfelt]

If the file is too big to write to a single jpg, rsgpr unhelpfully fails with this error:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Parameter(ParameterError { kind: DimensionMismatch, underlying: None })', src/gpr.rs:1517:42
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Either it should fail gracefully (no panic, keep on going with a warning saying "can't write jpg because it's too big"), or multiple jpg files should be written.