title: Erase using brush in GLPaint
body: <p>As part of modifying the <code>GLPaint</code>, I am trying to add erase functionality where user could select an eraser button and erase the painted area just as painting. </p>
  
  <p>I am trying have a conditional statement within <code>"renderLineFromPoint:(CGPoint)start toPoint:(CGPoint)end"</code> method so that I could check whether the stroke is for painting or erasing. </p>
  
  <p>For erasing I do not know how to make use of the "<code>start</code>" and "<code>end</code>" parameters for erasing. Is there any method call in OpenGL like <code>glClear()</code> that accepts these two parameter and does erase?</p>
  
  <p>Any pointer will be very helpful. Thank you.</p>