avcodec/libwebpenc: add quality option

previously quality could only be set through qscale/global_quality but the scale
was inverted. Using a separate option avoids the confusion from qscale working
backward.

Reviewed-by: Benoit Fouet <benoit.fouet@free.fr>
Reviewed-by: Clément Bœsch <u@pkh.me>
Reviewed-by: Nicolas George <george@nsup.org>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>