Revert "swscale: update context offsets after removal of AlpMmxFilter."
(commit a95e3fa90b4190381b65d180eec5a4027075e2da)
and
Revert "swscale: Remove some write-only variables related to alpha handling."
(commit 9d03cb9fc5ddf914920ab0dbe13f19a34c754966).
They broke alpha handling - it's the evil inline asm that still uses that
variable, so it's not truely write-only.