Fix encoder mem allocation when picture size is changed

After the picture size was changed to a bigger one, the internal memory was
corrupted and multithreaded encoder was deadlocking.

Memory for last frame's MVs, segmentation map and active map were allocated when
the compressor was created (vp8_create_compressor). Buffers need to be
reallocated when picture size is changed, so, the allocation was moved to
vp8_alloc_compressor_data, which is called every time the picture is resized.

Change-Id: I7ce16b8e69bbf0386d7997df57add155aada2240