Don't repeat alt+enter key event

Holding alt+enter should not continuously toggle fullscreen mode.

code/client/cl_keys.c

@@ -1239,6 +1239,11 @@ void CL_KeyDownEvent( int key, unsigned time )
 
 	if( keys[K_ALT].down && key == K_ENTER )
 	{
+		// don't repeat fullscreen toggle when keys are held down
+		if ( keys[K_ENTER].repeats > 1 ) {
+			return;
+		}
+
 		Cvar_SetValue( "r_fullscreen",
 			!Cvar_VariableIntegerValue( "r_fullscreen" ) );
 		return;