imageop: tune up X-Trans bilinear downsample

This improves on 6a5ace89d2b1ebdc308975db7d72a9ed7a7363d0 which made bilinear downsample-demosaiced images slightly softer. Results should also be sharper than those produced by dt_iop_clip_and_zoom_demosaic_third_size_xtrans() prior to that commit. Also, fixes long-standing bug which offset bilinear images up/left slightly compared to VNG/Markesteijn results.