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.h

@@ -234,6 +234,8 @@ class editorclass{
   //Direct Mode variables
   int dmtile;
   int dmtileeditor;
+
+  int returneditoralpha = 0;
 };
 
 void addedentity(int xp, int yp, int tp, int p1=0, int p2=0, int p3=0, int p4=0, int p5=320, int p6=240);