1. canvas.saveLayer(top_rectangle)
 2. canvas.saveLayer(bottom_rectangle) // Now the current layer.
 3. canvas.drawXYZ() calls from children
 4. canvas.drawRectangle(top_rectangle) // Completely outside area of current layer.
 5. canvas.drawRectangle(bottom_rectangle) // Bullseye on current layer.
 6. canvas.restoreToCount(beginning)