Corrected color bug with SaveAsJpeg on Windows Phone 8

Adjusted the Windows Phone 8 version of Texture2D's SaveAsJpeg class to
switch the pixels RGBA/BGRA values so that Writeablebitmap will read the
data correctly.  Added helper function in Texture2D called ConvertToRGBA
to convert the pixel data.  Images now save as properly colored JPEGs.
Moved pixel format conversion off "UI Thread".
Removed excess assignment of the G and A Pixels.
Merge pull request #2029 from Strifex/develop

Corrected color bug with SaveAsJpeg on Windows Phone 8