ofxsUnPremult: When treating RGB set alpha to 1.
Explanation: Some effects set the Unpremult parameter to true even if
the input stream is opaque, as a result of this the
ofxsUnPremult/ofxsPremult function are called, but since theres no
alpha, everything becomes black.

2 solutions: either set alpha to 1 in ofxsUnpremult or never set to
true the premult parameters for all plugins when input stream is opaque