See the complaint on #4039. This doesn't fix that particular
issue (it's somewhat unrelated), but does solve around
a file parsing problem reported by @void-in
Full details of the encoder are in the detailed description in the
source itself. But this is effectively an "optimised" SUB encoder
which is similar to the add_sub encoder except it doesn't bother to
use the ADD instructions at all, and it doesn't zero out EAX for
each 4-byte block unless absolutely necessary. This results in
payloads being MUCH smaller (in some cases 30% or more is saved).