libvpx

Improve scale_factors struct

The ref's scale_factors are set at frame level, and then copied for
each partition block. Since the struct members are mostly constant,
this patch separated the constant and non-constant members, and
reduced struct copying. This gave 0.5% ~ 1.4% decoder speed gain.

Change-Id: I94043bf5a6995c8042da52e5c661818dfa6f6d4c