Unifying tile decoding for both direct and inverse tile order.

Now tile decoding consists of two stages:
1. Find tile buffer start and its size, put this info into tile_buffers.
2. Decode each tile based on information from tile_buffers.

It seems that stage 1 can also be reused by multithreaded tile decoder.

Change-Id: If0cdaefdd6d10bb41c63561346c9ae4cfac081dd