Set currentAlpha to 1 if showAlpha is off when dragging in slider or dragger

This handles the case where I want to support hex colors + "transparent".

Spectrum handles setting the value to "transparent" fine, but then there's no way to change the alpha back to 1 with the alpha slider disabled. This will set the currentAlpha to 1 if you drag on the slider/dragger and the alpha slider is disabled.