encoder: Fix scale_and_extend_frame() for non-4:2:0
encoder: Fix scale_and_extend_frame() for non-4:2:0

The code to pre-scale reference frames was hardcoded to only work
for 4:2:0. This makes it work for 4:2:2, 4:4:0, and 4:4:4, also.

It also eliminates the special case for a fourth plane, based on the
assert above that the total number of planes is no more than three.

Change-Id: I79870d2d11bfc3b6e2e73c48a9ba679bffab186f

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom