glretrace: Use GL_TIMESTAMP (if available) for CPU profiling.
glretrace: Use GL_TIMESTAMP (if available) for CPU profiling.

The prior scheme of trying to continuously re-synchronize separate CPU
and GPU counters is doomed to always be unreliable. Instead, simply prefer
to use the GL_TIMESTAMP values for both GPU and for CPU profiling.

v2: José Fonseca: use glGet(GL_TIMESTAMP)

EoD/apitrace,trtt/apitrace,swq0553/apitrace,PeterLValve/apitrace,tuanthng/apitrace,schulmar/apitrace,surround-io/apitrace,apitrace/apitrace,EoD/apitrace,surround-io/apitrace,surround-io/apitrace,tuanthng/apitrace,EoD/apitrace,swq0553/apitrace,tuanthng/apitrace,joshua5201/apitrace,trtt/apitrace,apitrace/apitrace,PeterLValve/apitrace,joshua5201/apitrace,trtt/apitrace,surround-io/apitrace,PeterLValve/apitrace,tuanthng/apitrace,schulmar/apitrace,PeterLValve/apitrace,EoD/apitrace,swq0553/apitrace,tuanthng/apitrace,apitrace/apitrace,joshua5201/apitrace,joshua5201/apitrace,EoD/apitrace,schulmar/apitrace,trtt/apitrace,schulmar/apitrace,swq0553/apitrace,schulmar/apitrace,surround-io/apitrace,joshua5201/apitrace,swq0553/apitrace,trtt/apitrace,apitrace/apitrace