WB-918 Transparency channel in captures: Additional opacity functions are now...

WB-918 Transparency channel in captures: Additional opacity functions are now enabled for separate RGB / Alpha blending using Develop Menu -> Separate RGB / Alpha Opacity.  Pixel RGBA is examined in View Mode using the pop-up menu.  The dialog shows pixel RGBA and if the format is pre-multiplied alpha, a second set of RGBA values are shown with pre-multiplication removed.