Track pipeline layout lifetime

[kvark]

Fixes #580
This one is interesting: it's not instantly destroyed when dropped, like bind group layouts. And it's not tracked for GPU usage like the resources. It's somewhat in-between. We have the following classes of tracking now:

1. no tracking, destroyed on drop. Applies to: bind group layouts, adapters
2. only CPU-side tracking. They go to "suspected" list on drop of self or anything that can point to them. Applies to: pipeline layouts
3. full GPU tracking. They go to "suspected" list on drop, then get associated with active submission before destruction. Applies to: buffers, textures, views, bind groups, pipelines

[grovesNL]

This makes sense to me, thank you!

[kvark]

Thanks for the review!
I think what the follow-up needs to be is us removing the class(1) of the tracking entirely. Everything then will be refcounted. Just some objects not caring about GPU usage, and others doing that.
bors r=grovesNL

[grovesNL]

Yeah that sounds good 👍 hopefully we can find some opportunities to simplify the tracking in general

[bors]

Build succeeded:

• continuous-integration/travis-ci/push