In the beginning of encoding and bitstream packing,
the frame context in common of codec is copied to the frame context
in each tile.
Initial prob and context is based on flat probability and
does not come from table at the moment.

The bd-rate change for the test set objective-1-fast
on awcy with high delay mode is:

PSNR   PSNR  HVS  SSIM  CIEDE 2000  PSNR Cb PSNR Cr MS SSIM
-0.77  -1.05      -0.74 -0.67       -0.67   -0.77   -0.88

Change-Id: Ic9105ac68aceb7486cb5f6f1c0b19df5853f2cb9