more accurate bilinear for int16 pixels
more accurate bilinear for int16 pixels

fixed-point interpolation could give small errors

see https://github.com/libvips/libvips/issues/1309

need a similar change for bicubic etc.

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