Fix tracing in non-main threads
This actually makes using COGL_TRACE_BEGIN*()
things work on any thread that registers itself with the MetaProfiler
thing (or the lower level Cogl stuff), which it didn't really before.
In the end, each trace mark will have a group name that contains the name of the thread; so far Sysprof shows traces as if they were all the same group in the timeline view, but they are distinguishable on the top panel. This is how it'll look: