Hide all the error-prone synchronization logic in the class, and expose a documented API.

This should have no impact on behavior - it was authored to make it harder to break things in the future.
That said, it appears to fix a whole bunch of flakes with the dEQP query tests.

Bug: b/133127573
Change-Id: I5c30b79b9b1cd36dba1fa2d3c34af0f5bd62772a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31816
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Ben Clayton <bclayton@google.com>