Direct, thread-safe, access to thread managed object

[ivovandongen]

This pr enables a thread to be suspended and the managed object to be accessed directly.

• Exposes thread.hpp
• Makes thread suspending/resuming thread safe so it can be called from arbitrary threads
• Adds a RAII class that blocks a thread and gives access to the object managed by it (BlockingThreadGuard)

Needed for:

• Android a-synchronous rendering: Android asynchronous rendering #9576

[jfirebaugh]

Makes thread suspending/resuming thread safe so it can be called from arbitrary threads

It doesn't look thread safe to me; there's even an assert(!paused) that will fail if a second thread tries to pause an already-paused thread.

[ivovandongen]

It doesn't look thread safe to me; there's even an assert(!paused) that will fail if a second thread tries to pause an already-paused thread.

Right. It was definitely missing some pieces. I've fixed up the pause/resume methods and added tests.

[jfirebaugh]

@ivovandongen This is no longer needed, correct?