Added an option for Premultiplied Alpha Blending

As per this conversation:

http://forum.processing.org/one/topic/drawing-opengl-into-pgraphics-with-alpha-0-5f-produces-extremely-faint-output-more-like-0-1f.html

...And this old bug report:
https://github.com/processing/processing/issues/2083.html

Sorry if this isn't how such simple suggestions are to be done, it's my
first time doing anything with git.