webp: Fix alpha initialisation

ff_get_format() in the next patch will reject formats which aren't in the
offered list, so the hack in 7cb9296db872c4221453e5411f242ebcfca62664 is
no longer valid.  Change the hack by adding a new field in the VP8 decoder
context to indicate that it's actually WebP and don't call ff_get_format()
at all in that case.
