Replace use of deprecated libvips conv method.
Replace use of deprecated libvips conv method.

Ensure unref of mask to fix minor memory leak of ~150 bytes/image.

kevinsawicki/sharp,philip8728/sharp,dtest/sharp,mdimitrov/sharp,mhirsch/sharp,papandreou/sharp,mcanthony/sharp,pporada-gl/sharp,mcanthony/sharp,lovell/sharp,kevinsawicki/sharp,pporada-gl/sharp,dtest/sharp,digital-flowers/sharp-win32,mdimitrov/sharp,philip8728/sharp,digital-flowers/sharp-win32,lovell/sharp,chrisriley/sharp,papandreou/sharp,cmtt/sharp,cmtt/sharp,philip8728/sharp,mdimitrov/sharp,leiyangyou/sharp,pporada-gl/sharp,cmtt/sharp,mdimitrov/sharp,dtest/sharp,papandreou/sharp,philip8728/sharp,chrisriley/sharp,brandonaaron/sharp,leiyangyou/sharp,brandonaaron/sharp,lovell/sharp,chrisriley/sharp,brandonaaron/sharp,digital-flowers/sharp-win32,kevinsawicki/sharp,dtest/sharp,papandreou/sharp,chrisriley/sharp,lovell/sharp,kevinsawicki/sharp,brandonaaron/sharp,philip8728/sharp,lovell/sharp,digital-flowers/sharp-win32,papandreou/sharp,kevinsawicki/sharp,digital-flowers/sharp-win32,leiyangyou/sharp,mhirsch/sharp,mcanthony/sharp,cmtt/sharp,leiyangyou/sharp,mcanthony/sharp,lovell/sharp,mhirsch/sharp,mcanthony/sharp,brandonaaron/sharp,digital-flowers/sharp-win32,pporada-gl/sharp,mhirsch/sharp,dtest/sharp,chrisriley/sharp,pporada-gl/sharp,digital-flowers/sharp-win32,mdimitrov/sharp,leiyangyou/sharp