Don't clobber default filters if env filters exist

[BigWingBeat]

Objective

LogPlugin has some default filters which suppress unhelpful noisy logging from dependencies. However, if the RUST_LOG env var is set, it completely discards these default filters in favour of whatever the user set, thus unleashing all of the spammy logging that is usually hidden.

Solution

Refactor the filter setup so that RUST_LOG filter configuration is applied on-top of the default filters, instead of entirely replacing them. The EnvFilter is constructed with the default filters, then any filters from RUST_LOG are parsed and added afterwards, so that they can selectively override the default filters, should the user desire that.

Testing

Manually tested with the logs example and various RUST_LOG values.