libvpx

Revert "Remove mi_grid_* structures."

(see I3a05cf1610679fed26e0b2eadd315a9ae91afdd6)

For the test clip used, the decoder performance improved by ~2%.
This is also an intermediate step towards adding back the
mode_info streams.

Change-Id: Idddc4a3f46e4180fbebddc156c4bbf177d5c2e0d