gltrace: flush trace buffer on createContext & makeCurrent

Certain apps (e.g. chrome) seem to create contexts which are unused
for long periods of time. If tracing is stopped before those contexts
are used, then the debugger never gets to know that these contexts
were created. Flushing the trace after these calls ensures that
the debugger knows about all created/used contexts.

Change-Id: I01baa11aa56ac89eddce3c2851e4bf01076984d1