Unlike the vertex/tiler jobs, which are different on 32-bit and 64-bit,
the fragment payload is simple enough that it's the same, so there's an
extra 32 bits of padding after the next-job pointer on 32-bit.
Previously, that padding was hard-coded into the payload structure, but
we don't want it to exist on 64-bit, since then the payload structure
would have to be different on 32 and 64 bit for no good reason. Just
make the decoder adjust the padding itself.