Fix wrong viewport in Composer and gl.Clear(0) error

The viewport was correctly set on camera but then was erased by camera.setStateSet( element.filter.getStateSet() )
The correct behaviour is to set the viewport _after_ copying the filter's stateset.
Also setting clearMask at 0 leads to "Error: WebGL: Calling gl.clear(0) has no effect." on FF
Use the Composer.js from the source tree