Use .clear() when removing text boxes in reset functions

graphics.textbox.clear() should be used instead of
graphics.textboxremove() or graphics.textboxremovefast(), because even
with graphics.textboxremovefast(), you'll still have to process at least
one frame of GAMEMODE logic before the text boxes are actually properly
removed, and this caused a 1-frame glitch when exiting playtesting with
text boxes on-screen and then re-entering playtesting.

Technically I could've only fixed it in Game::returntoeditor(), but I
wanted to be safe, so I also fixed it in scriptclass::hardreset(), too.

desktop_version/src/Game.cpp

@@ -7298,7 +7298,7 @@ void Game::returntoeditor()
 {
     game.gamestate = EDITORMODE;
 
-    graphics.textboxremove();
+    graphics.textbox.clear();
     game.hascontrol = true;
     game.advancetext = false;
     game.completestop = false;

desktop_version/src/Script.cpp

@@ -3648,7 +3648,7 @@ void scriptclass::hardreset()
 
 	//dwgraphicsclass
 	graphics.backgrounddrawn = false;
-	graphics.textboxremovefast();
+	graphics.textbox.clear();
 	graphics.flipmode = false; //This will be reset if needs be elsewhere
 	graphics.showcutscenebars = false;
 	graphics.cutscenebarspos = 0;