Currently in debug builds swiftshader is not thread safe because of this
tracking code. This change makes it safe to create/delete objects from
multiple threads.

Change-Id: I9c8d3efc370687891b1acc786499bda02c86ad01
Reviewed-on: https://swiftshader-review.googlesource.com/10528Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Vanik <benvanik@google.com>