Add rounded macron-mask image

Done with:

convert img/macron-mask.png \( +clone  -alpha extract -draw 'fill black polygon 0,0 0,15 15,0 fill white circle 15,15 15,0' \( +clone -flip \) -compose Multiply -composite \( +clone -flop \) -compose Multiply -composite \) -alpha off -compose CopyOpacity -composite  img/macron-mask-round.png
Status Job ID Name Coverage
  Test
passed #719964
html-validity-check

00:00:18

passed #719965
json-validity-check

00:00:18