QGtkHelpers: Fix QImage -> GdkPixmap conversion for all cases

If this isn't in one of the two correct formats, make it so. Also check
that the image actually has alpha before telling GdkPixmap it does.

This fixes copying images in some cases having a whacky color in
Telegram Desktop.

src/qgtkhelpers.cpp

@@ -26,16 +26,26 @@
 
 #include "qgtkhelpers.h"
 
-QGtkRefPtr<GdkPixbuf> qt_imageToPixbuf(const QImage &image)
+QGtkRefPtr<GdkPixbuf> qt_imageToPixbuf(const QImage &cimage)
 {
-    if (image.isNull())
+    if (cimage.isNull())
         return 0;
+    QImage image = cimage;
+    if (image.hasAlphaChannel()) {
+        if (image.format() != QImage::Format_RGBA8888) {
+            image = image.convertToFormat(QImage::Format_RGBA8888);
+        }
+    } else {
+        if (image.format() != QImage::Format_RGB888) {
+            image = image.convertToFormat(QImage::Format_RGB888);
+        }
+    }
     guchar *buf = (guchar*)malloc(image.byteCount());
     memcpy(buf, image.constBits(), image.byteCount());
     QGtkRefPtr<GdkPixbuf> gpb = gdk_pixbuf_new_from_data(
         buf,
         GDK_COLORSPACE_RGB,
-        true, // ### assert image is in RGBA
+        image.hasAlphaChannel(),
         8,
         image.width(),
         image.height(),