Only log warnings and above, by default

[hannobraun]

Since I re-enabled logging recently, there's a lot of noise due to info-level logging from wgpu. I think we should only log warnings and errors by default, while still respecting whatever the user specifies via RUST_LOG. This would involve making the logger initialization in main.rs more intelligent, although I'm not completely sure what needs to be done.

Labeling https://github.com/hannobraun/Fornjot/labels/good%20first%20issue, as this is mostly a Rust/Tracing issue that doesn't require advanced knowledge of Fornjot.