Auto min and max partition size experiment.

Speed feature experiment to set an upper and lower
partition size limit based on what has been seen
in spatial neighbors.

This seems to gives quite reasonable speed gains in local
(10-15%) and when used with speed 0 the losses are small
(0.25% derf, 0.35% stdhd). However, for now I am only
enabling it on speed 1 as there may be clashes with the existing
temporal partition selection in speed 2.

Using a tighter min / max around the range derived from the
neighbors increases speed further but at the cost of a
bigger quality loss. However,  I think this spatial method could
be combined with data from either the last frame or a variance
method (or both) to refine the range of minimum and maximum
partition size. I.e. consider the min and max from spatial and
temporal neighbors and the variance recommendation.

Change-Id: I1b96bf8b84368d6aad0c7aa600fe141b4f07435f