Unify View Layers

All of visible layers unify to single layer when press button on the layer window.

From bottom(below) layer to top(above) layer, visible(no-transparent) pixel is override by pixel in above layer.

Unifying is NOT depended on 'Mask(in palette auxiliary attribute)', 'Not use(in palette auxiliary attribute)' and 'Editable(in layer attribute)'. Target of unifying is all of 'Visible(in layer attribute)' layers.