Ensure oldfadeamount is updated when fadeamount is

Like cutscene bars, I've added Graphics::setfade(), to ensure that no
deltaframe rendering glitches happen due to oldfadeamount not being
updated properly.

And indeed, this fixes a deltaframe rendering glitch that happens if you
return to the editor from playtesting on a faded-out screen, then fade
out again (by either re-entering playtesting and then cause a fadeout to
happen again, or by quitting from the editor afterwards). The same
glitch also happens outside of in-editor playtesting if you exit to the
menu while the screen is faded out.

desktop_version/src/Script.cpp

@@ -1498,7 +1498,7 @@ void scriptclass::run(void)
 			}
 			else if (words[0] == "befadein")
 			{
-				graphics.fadeamount = 0;
+				graphics.setfade(0);
 				graphics.fademode= 0;
 			}
 			else if (words[0] == "fadein")