indeo3enc: rework the logic with motion cells

indeo3enc: rework the logic with motion cells

Previously motion vectors were ranged by the number of cells using them and
if the vector outside the allowed number of MVs was close enough to
a popular one the latter was used instead.

Now the MVs are ranked by the total area of cells using them and the vectors
not in the list are pruned entirely. Cells with those vectors are replaced
with the intra-coded ones.