Given the largest transform size is 32x32, this commmit changes size
estiiation based on the size rounding up to 32 multiples to avoid
insufficient buffer allocations.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=36

Change-Id: I6eab09dc6acdc0f5a6bcadb918d62c4852aae21f