thread_utils: release mutex before signaling
thread_utils: release mutex before signaling

holding the associated mutex while signaling a condition variable isn't
necessary and in some implementations will reduce performance as the
woken thread may test the mutex, fail and go back to sleep.

Change-Id: Id685a47b0c76fc4a1c5acedcb6623e8c55056415

PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp