wrap Arc<Backtrace> in Option, saving memory when backtraces are disabled

[Firestar99]

This PR wraps all Arc<Backtrace> in Option, as to not have to allocate an Arc of a disabled Backtrace each time GPU memory is allocated with backtraces are disabled. This is probably a premature optimization, but it was so easy to implement I couldn't resist :D

I only tested the vulkan implementation and visualizer on Linux, I hope CI catches issues for the other platforms.

[manon-traverse]

Looks like a nice small optimization. Thanks for your contribution! :D

[MarijnS95]

I recall checking long ago that Backtrace::Captured { .. } used to be quite small back in the day, is that no longer the case?

Regardless, this saves us some bytes in the Arc too so I guess it's nice to have. There's nothing to do about the ambiguity between None and Some(Arc::new(Backtrace::Disabled)) but it is what it is :)

[MarijnS95]

Looking at this PR I still wonder why the Backtrace is wrapped in an Arc; in e45e25a I wrote that that is to support "Cloneing" the Backtrace, but glancing at the implementation that is only because of mistakes in the ownership transfer in our fn allocate() code and copies for AllocationReport which should either not receive a Backtrace at all, or borrow it temporarily while the caller can view the backtrace.

[Firestar99]

why the Backtrace is wrapped in an Arc

I don't have the time right now to investigate that further, feel free to do that yourself or leave the PR as is.