Sequencer: expose GL preview alpha in scene UI

While this isn't essential, accessing this setting required navigating to each scene and using render menu.
Expose in sequencer UI for more convenient access.

release/scripts/startup/bl_ui/space_sequencer.py

@@ -866,6 +866,14 @@ class SEQUENCER_PT_scene(SequencerButtonsPanel, Panel):
         if scene:
             layout.prop(scene, "audio_volume", text="Audio Volume")
 
+        if not strip.use_sequence:
+            if scene:
+                # Warning, this is not a good convention to follow.
+                # Expose here because setting the alpha from the 'Render' menu is very inconvenient.
+                layout.label("Preview")
+                layout.prop(scene.render, "alpha_mode")
+
+        if scene:
             sta = scene.frame_start
             end = scene.frame_end
             layout.label(text=iface_("Original frame range: %d-%d (%d)") % (sta, end, end - sta + 1), translate=False)