alpha: preparatory cleanup

* make ALPHNew/Delete static
* properly init ALPHDec::io_
* introduce AllocateAlphaPlane() and WebPDeallocateAlphaMemory()
* reorganize VP8DecompressAlphaRows()

but we're still allocate the full alpha-plane. Optim will come
in another patch since it's tricky

Change-Id: Ib6f190a40abb7926a71535b0ed67c39d0974e06a
4 files changed