This patch slightly reorders the fields in yv12_buffer_config and then
uses anonymous unions in order to make it possible to write code that
iterates uniformly over planes.

The patch also ports some code (mostly in yv12extend.c and
aom_scale.c) to show how this can make things more concise.

This should make no difference to the coded results. I think it's
unlikely to have any significant performance impact (the reordered
fields in a yv12_buffer_config only come to 17*4 = 68 bytes in total,
so almost fit in a normal sized cache line).

Change-Id: Iebb46344500b9df82915f34cfd193e189d712062