Fix convolution vector path posterization
Fix convolution vector path posterization

This fixes posterization in the vector path for convolution caused by a
typo in commit eefb2dad980c12922ad15d987be7c4b42a44d5cf.

The fix was suggested by @jcupitt.

lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips