mt trace: add helper to track GL contexts on a per-thread basis

mt trace: add helper to track GL contexts on a per-thread basis

This is needed for muti-threaded tracing to work properly.

Tracing multi-threaded GL apps requires that we track the lifetime of
the GL context objects and store thread specific state like the currently
active GL context on thread-local memory area.

Also add 'retain_count' to the context needed by the CGL retain/release
context calls.

Needed by the 4 upcoming GL API specific patches.

Signed-off-by: Imre Deak <imre.deak@intel.com>