int x1,x2,y1,y2;
boolean esperando;
void setup(){
  size(400,400);
  esperando = false;
}
void loop(){
  if(esperando){
    line(x1,y1,mouseX,mouseY);
  }
}
void mousePressed(){
  x1=mouseX;
  y1=mouseY;
  esperando = true;
}
void mouseReleased(){
  x2=mouseX;
  y2=mouseY; 
  rectMode(CORNERS); 
  rect(x1,y1,x2,y2);
  esperando = false;
}