Fix: Qt failing to paint a darker button if source image is indexed

By converting the image to an 8-bits per channel image with alpha channel
we make sure the client won't fail painting a darker button,
and keep the transparency if the source image had.

src/aoemotebutton.cpp

@@ -27,6 +27,7 @@ void AOEmoteButton::set_image(QString p_image, QString p_emote_comment)
   }
   else if (p_image.contains("_on") && file_exists(tmp_p_image.replace("_on", "_off"))) {
     QImage tmpImage(tmp_p_image);
+    tmpImage = tmpImage.convertToFormat(QImage::Format_ARGB32);
     QPoint p1, p2;
     p2.setY(tmpImage.height());