Make "[Press ENTER to return to editor]" fade out after a bit

This makes the "[Press ENTER to return to editor]" fade out after a few frames, allowing screenshots of custom levels to be cleaner and to make sure nothing is obscured while the user is editing their level.
This commit also adds alpha support in BlitSurfaceColoured, where it takes into account the alpha of the pixel *and* the alpha of the color.
`graphics::getRGBA(r,g,b,a)` was added to help with this.

desktop_version/src/editor.cpp

@@ -4575,6 +4575,7 @@ void editorinput( KeyPoll& key, Graphics& dwgfx, Game& game, mapclass& map, enti
 
                         music.stopmusic();
                         dwgfx.backgrounddrawn=false;
+                        ed.returneditoralpha = 1000; // Let's start it higher than 255 since it gets clamped
                         script.startgamemode(21, key, dwgfx, game, map, obj, help, music);
                     }
                     //Return to game