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/Graphics.h

@@ -45,6 +45,7 @@ public:
 	void drawmenu(int cr, int cg, int cb, bool levelmenu = false);
 
 	void processfade(void);
+	void setfade(const int amount);
 
 	void drawfade(void);