This will fix the frame parallel decode hang on windows
due to not enough semaphores.
This will also make the frame parallel decode safer as
the number of frame buffers could only support maximum
8 threads.
Change-Id: Id9ef50692819dcbebbd74a0aabffbfb3f39a4309
Prepare for frame parallel decoding, the reference count buffers
need to be protected by mutex. Move vp9_thread.* to common
folder so that those buffers could use cross-platform mutex
from vp9_thread.*.
Change-Id: I541277cf15eefed6641555944f67f4a0bcdc8154