libvpx

Reduce max_partition_size for low resolutions at speed 5.

For speed 5 real-time mode, the selection of the partition size for
superblocks on the segment (aq-mode=3) uses the non-rd recursive
pick partition search, and can sometimes select 64x64.

For low resolutions, visually better to limit this to 32x32.

Change-Id: I69657a7ed8899f8b3cf8c9c318a2509c5c72c565