added option to specify preserveDrawingBuffer for "painting" or "ghost trails"

set preserveDrawingBuffer in PIXI object
check render function, when preserveDrawingBuffer is true the screen will not be cleared automatically
added clear function to WebGLRenderer, user may want to clear periodically

*** please test

TO DO: alpha for background clearing to allow for "ghost trails"? Don't know how this works...

I wanted to paint with your library, now I can, thanks :dart: