Grayscale PNG with QImage does not support transparency

Creating an 8-bit greyscale PNG does not support setting a transparency using the tRNS field in the PNG header.  The work around is to write the PNG using QImage::save, then read the entire file to QByteArray, insert the tRNS field, and then write the file.  It should be fairly straight forward to allow the user to set a transparency before creating the image such that the header is included when invoking QImage::save.