This will help make sense of vblank events in a multi-screen
environment (where there are two streams of vblanks), and hopefully
also useful for correlating to compositor events.
If we want to start watching other perf events, we need a way to look
up what perf event we got when a PERF_RECORD_SAMPLE comes in. The
sample identifier is what does that.
The lib/ directory was getting a bit out of hand, so this tries
to organize things a bit so it is easier going forward to locate
the code people want to patch.
