Two-steps scaling in VP9 encoder dynamic resizing.

Dynamic resizing now support two-steps scaling: first go down to
3/4 and then 1/2. This feature is under a flag which controls the
switch between two-steps scaling and one-step scaling (1/2 only).

Change-Id: I3a6c1d3d5668cf8e016a0a02aeca737565604a0f