The aomenc API is modified to allow --tile_width and --tile_height to take a list of sizes.
For example, --tile_width=2,3 will give tiles of widths 2,3,2,3,... until the image width
is exhausted. --tile_width=2 will still work to gives tiles of width 2.

Change-Id: I2afa14d404557aa8b7341b20f7477590e03e0bdb