libvpx

Skip the partition search for still frames

This patch re-enabled the feature in Pengchong's patch
(commit 12861260732a4fd5f6b667ce9d5105dc9b606eda). Originally, it
was turned on while use_lastframe_partitioning > 0(not used anymore).
Now it was added as a feature, and turned on while speed >= 2.
As described in the original patch, this feature helps speed up the
slideshows in YouTube.

Change-Id: I1b0f18d65da1ee1c8d1e117dabba910c5207c471