Fix rectangular partition check in speed 1

Make encoder skip rectangular partition check in speed 1 and above,
when early termination was triggered in partition split.
Thanks Guillaume (gmartres@) for catching this issue.

This change makes bus_cif at 2000kbps speed 1 runtime goes down from
25612ms to 23438ms (about 9% speed-up), at the expense of -0.235%
performance down.

Change-Id: I98613fad081a261d30d5fa206f934ca70601c180