vp9: address integer sanitizer warning
vp9: address integer sanitizer warning

Comparing the size values with subtraction requires casting. Sort in
descending order.

(a < b) - (a > b)
If a is greater, this is 0 - 1 = -1
If the  values are equal, this is 0 - 0 = 0
If b is greater, this is 1 - 0 = 1

Change-Id: I5c20fd10fbc97c391c6858235c44d25d7db57f0e

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