avcodec/asvenc: Fix integer overflow in level

Warn if the qscale is too low for the input data and clip levels to
minimize artifacts

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/asvenc: fix AAN scaling

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/webp: fix default palette color 0xff000000 -> 0x00000000

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>