libvpx

Added a speed feature "allow_skip_recode"

This commit added a speed feature to make the logic of calculating
skip_recode on a block level more explicit. This also enable the
feature to be enabled at speed 5 where the previous logic is too
conservative, help gain back the lost speed for --rt(-5).

Change-Id: Ieb37ca3e85c2e7bda343486edf13d5f5395f2233