Remove the single_threaded/multi_threaded feature flags

[emilk]

Always use parking_lot for mutexes, i.e. always be multi-threaded.

Closes #1379

Please take a look @DusterTheFirst

cargo bench shows no difference, so at this point I see no real reason of keeping the single_threaded path.

There is an alternative solution (which is orthogonal to this one):

• Put repaint callback and tex manager behind separate mutexes #1389

which has the advantage of not triggering the problem caught in #1380

[DusterTheFirst]

This looks to be a better solution since it un-complicates the single/multi-threaded feature situation while at the same time avoiding the performance cost sen in #1389.

I do like the switch to atomics for handling request_redraw, avoiding the need for a lock, seen in #1389. This change may be worth adopting in this PR as well.

To address #1380, this PR could be paired with the change you mentioned in your comment (#1380 (comment)).