Fix clippy warnings

[MarcoIeni]

Right now some clippy warnings are allowed in the code. They look something like this:

delta/src/paint.rs

Line 243 in 15d06cb

#[allow(clippy::too_many_arguments)]

So just search for the word "clippy" in the code and you will find them.
You should do the following:

1. run cargo clippy and check that there are no warnings
2. delete the allow line
3. run cargo clippy and check that you have a warning now
4. try to fix the warning

The only one you should not remove MAYBE it's #[allow(clippy::tabs_in_doc_comments)]. But I am not sure about this.