According to the WebP Lossless Bitstream Specification
"each transform is allowed to be used only once".

If a transform is more than once this can lead to memory
corruption.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit c089e720)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>