Opacity slider cannot work with preventDefault

The opacity slider cannot work if we call preventDefault on touchmove. We used to call preventDefault to prevent horizontal scrolling, but we're not able to reproduce the problem.
Casts for opacity values

This fixes the opacity sliders on iOS. We pass Number values to OpenLayers, and String values to the browser.
Merge pull request #486 from geoadmin/fix_opacity-slider-ios

Opacity slider can’t be changed on mobile