Fix centering of game complete texts if level font != interface font

Textboxes created with graphics.createtextboxflipme() use PR_FONT_LEVEL
by default, but can be overridden with graphics.textboxprintflags() to,
for example, set PR_FONT_INTERFACE. This happens for the textboxes on
the Game Complete screen, which use interface text. The textboxes are
centered by setting the X position to -1 though, which means they're
solely centered based on the width of the first line, in the level
font (because the font hasn't been changed to the interface font yet).

Normally, this isn't a problem, because in the main game (where the
Game Complete screen usually appears), the level font is always equal
to the interface font. However, in custom levels you can still get it
(by calling gamestate 3500) and in that case some of the text may be
misaligned. This change fixes that by adding graphics.textboxcenterx()
to these textboxes.

As far as I can tell, these are the only textboxes that are centered
by just x=-1 despite changing the font afterwards.

desktop_version/src/Game.cpp

@@ -2881,6 +2881,7 @@ void Game::updatestate(void)
 
             graphics.createtextboxflipme(loc::gettext("All Crew Members Rescued!"), -1, 64, TEXT_COLOUR("transparent"));
             graphics.textboxprintflags(PR_FONT_INTERFACE);
+            graphics.textboxcenterx();
             char buffer[SCREEN_WIDTH_CHARS + 1];
             timestringcenti(buffer, sizeof(buffer));
             savetime = buffer;
@@ -2956,11 +2957,13 @@ void Game::updatestate(void)
             );
             graphics.createtextboxflipme(buffer, -1, 158, TEXT_COLOUR("transparent"));
             graphics.textboxprintflags(PR_FONT_INTERFACE);
+            graphics.textboxcenterx();
             graphics.createtextboxflipme(
                 loc::gettext_roomname(map.custommode, hardestroom_x, hardestroom_y, hardestroom.c_str(), hardestroom_specialname),
                 -1, 170, TEXT_COLOUR("transparent")
             );
             graphics.textboxprintflags(PR_FONT_INTERFACE);
+            graphics.textboxcenterx();
             break;
         }
         case 3508: