vp9: Reduce stack usage of choose_partitioning.
vp9: Reduce stack usage of choose_partitioning.

Move vt2 to heap.
Reduce the stack usage from ~87K to ~44K.

BUG=b/68362457

Change-Id: I8f5f93712934d59a8cc4564378172d409a736a2e

ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx