Fix for fetching "dll export" functions via getprocaddress on linux
Fetch render state after making replay context current
Serialise out calls to MakeCurrent mid-frame-capture.

* We need to serialise out the new state vector that's applied when we
  change context.
* Note we still have very hacky incomplete support of contexts, and no
  support of true threading. We currently assume all contexts share with
  all others, and if any actual parallel rendering happened it would break.