Mutex-protect the ring buffers (as we do in the *nix builds)

This fixes an observed crash in sexyz for Windows. The crash was in RingBufWrite() where 'remain' was set to -1 (4.2B) apparently because pHead and/or pTail changed (to pTail=pHead) during the invocation of the RINGBUF_FILL_LEVEL() macro, because it wasn't thread-safe.

Amazing this problem never came up before.