Adjust porta snap

[yuxshao]

porta input is a bit weird in that you have to drag half a quantize length to get a full quantize-length porta event. other inputs give you a full quantize-length event as soon as you click. the reason porta is different is you may want a 0-length porta; but in collage that's accomplished if you simply don't move your mouse. that's more natural than what's currently done here.