TileByteStream: Removing more MT-unsafe mutable data from TileROD_Decoder.

Merged Scott Snyder requested to merge ssnyder/athena:const5.TileByteStream-20190102 into master

Remove m_pRwChVec, m_cell2Double. Create these objects on the stack where needed. Change from a vector of pointers to a flat vector to remove most of the allocation/deallocation overhead (which is probably why they were members in the first place).

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