If configured with --enable-ext-tile, the codec uses an alternative
tile coding syntax in the bitstream. Changes include::
 - The maximum number of tile rows and columns is extended to 1024
   each.
 - The minimum tile width/height is 64 pixels (1 superblock).
 - A tile copy mode is added where a tile directly reuse the coded
   data of a previous tile
 - The meaning of the tile-columns and tile-rows codec parameters are
   overloaded to mean tile-width and tile-height in units of 64
   pixels.
 - All tiles should now be independent, including rows within the
   same columns, so large scale parallel, or independent decoding is
   possible.
 - vpxdec also gained the options to decode only a particular tile,
   tile row, or tile column.

Changes without --enable-ext-tile:
 - All tiles should now be independent, including rows within the
   same columns, so large scale parallel, or independent decoding is
   possible.
 - vpxenc default tile configuration changed to use 1 tile column.

Change-Id: I0cd08ad550967ac18622dae5e98ad23d581cb33e