Hierarchical motion estimation (#673)

* Generate downsampled versions of input frames and store them with reference frames

* Add hierarchical ME, where initial ME is done using 4x subsampled 64x64 blocks. MVs are refined using 2x subsampled 32x32 blocks. In the refinement step, search is done around MV found for colocated 64x64 block, and also around MVs found for neighboring 64x64 blocks