Use PImage to save state of image currently on screen

[eoinfennessy]

The method used to save image on right click is currently a little bit of a hack - it'll always save the last image to be displayed before quitting the program, even if the user has not tried to save an image. This was necessary due to a constraint in the spec for this assignment that ruled out the use of PImage, and also because calling the saveImage() method using a mouse click or key press saves the next frame to be displayed instead of the one currently on screen.

TODO:

• Replace save frame method with one to copy the state of the currently displayed image to a PImage object
• Change mousePressed() method to save the PImage to disk after prompting user for filename