GstBufferPool implementation was referenced for this GstD3D11PoolAllocator,
for example GstAtomicQueue, various atomic operations, and GstPoll ones.
However, such combination seems to be almost pointless
since gst_poll_{read,write}_control() takes mutex and also
GstPoll uses Win32 event handle internally.
Use simple SRWLOCK and CONDITION_VARIABLE instead, and don't make things
complicated/inefficient.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2887>
76d1257cd3