Some experiments with XIO and webworkers.
Remove obsolete check on the DOM element in X.interactor.
Only add a null check when assigning a dom element to an interactor.
Export several properties of X.texture and X.slice.
Reset flag for mousewheel observation.
Add the X.texture.grayscale flag to support 1 channel textures without overhead of mapping to RGBA.
Merge branch 'webworker_xp'