Fixed the ignoring of the alpha channel in WinRT

Not ideal, because it hardcodes `Premultiplied`, when the image format's alpha could potentially be `Straight`. However, this is still better than `Ignore` as it was before. It was because  `decoder.BitmapAlphaMode` would return Ignore since Microsoft just sets it to Ignore regardless of format.

http://social.msdn.microsoft.com/Forums/windowsapps/en-US/454e63bf-46f8-43b3-b8bd-d9280fb49d40/how-to-get-the-right-alpha-mode-when-editing-an-image-using-bitmapdecoder?forum=winappswithcsharp