Instead of creating a circular mask by drawing a circle in processing, try creating an actual image in another program (ie photoshop, gimp) with the two circles as you would like.  Then, mask the first image with the two circle image and set the location of the mask image to your mouseX, mouseY.  While the circles should be the actual size you want them, you'll have to make the rest of the mask image 4x the size of the actual image. Otherwise, when you move the circles to say, the upper right hand corner, your mask won't cover all of the image anymore.