Profiler can't be used accross threads or with interleaved scopes

[Wumpf]

By design the user is currently forced to use one profiler per thread. In fact even working with two command encoders on a single thread in an interleaved fashion is not possible right now since every call to end_scope needs its corresponding call to begin_scope to be in correct order.

begin_scope needs to return a handle that end_scope works with instead. Also, both functions need to work with interior mutability, each requiring &GpuProfiler instead of &mut GpuProfiler.
I believe forcing &mut GpuProfiler for all other methods (query resolve, processing frame etc.) still makes sense though since this greatly simplifies things both for implementation and potential error cases

[Wumpf]

(work in progress)