Make SdlDrop and SubsystemDrop safer to use

[daggerbot]

Fixes #1247 and #1248. Ensures better initialization/drop guarantees for SdlDrop and SubsystemDrop by using global reference counts. This has a few caveats:

• SdlDrop can no longer be initialized from outside the crate::sdl module. Any external code depending on this (hopefully there was none because this is extremely unsafe) will break.
• Public but hidden functions returning SdlDrop and SubsystemDrop are no longer wrapped in Rc<>. Any external code depending on these functions will break. It would be possible to reverse this change, but with global reference counting, there is no longer any good reason to do so.
• The fixes caused at least one of the tests to break because tests are run in parallel by default, and more than one test initializes SDL. Now that this can only be done from one thread, one must specify --test-threads=1 for the tests to pass. Maybe this was already the case, but I didn't check before making these changes. Example: cargo test -- --test-threads=1

Because the first two caveats are API breaks, even if hidden from documentation, I recommend a version bump to 0.36.0 if these changes are accepted.

[Cobrand]

Nice job!

The fixes caused at least one of the tests to break because tests are run in parallel by default, and more than one test initializes SDL. Now that this can only be done from one thread, one must specify --test-threads=1 for the tests to pass. Maybe this was already the case, but I didn't check before making these changes. Example: cargo test -- --test-threads=1

This was already done in Github Actions for the tests, so no worries about that.

Could you add a line in the changelog to mention your changes? It's internal but who knows, it might break someone's code.

[daggerbot]

Could you add a line in the changelog to mention your changes? It's internal but who knows, it might break someone's code.

Done. Do you want the changelog addition squashed, or is it fine like this?

[Cobrand]

Thanks!