libvpx

Add segmentation map array for current and last frame segmentation.

The original implementation only allocates one segmentation map and this
works fine for serial decode. But for frame parallel decode,  each thread
need to have its own segmentation map and the last frame segmentation map
should be provided from last frame decoding thread.

After finishing decoding a frame, thread need to serve the old segmentation
map that associate with the previous decoded frame. The thread also need to
use another segmentation map for decoding the current frame.

Change-Id: I442ddff36b5de9cb8a7eb59e225744c78f4492d8