color: when syntax coloring is toggled back on, calculate multiline data

This will not catch all cases, but at least it will precalculate the
data when syntax coloring was toggled off (M-Y) while in one buffer
and then toggled back on after having switched to a new buffer.

src/nano.c

@@ -1092,6 +1092,7 @@ void do_toggle(int flag)
 			break;
 #ifdef ENABLE_COLOR
 		case NO_SYNTAX:
+			precalc_multicolorinfo();
 			refresh_needed = TRUE;
 			break;
 #endif