This variable can be used to find out if a particular view port is currently visible or not. You can also set this variable to effectively turn "on" or "off" a view by setting the value to true (visible) or false (invisible). Note that even if you have a view port set to visible, if view ports are not enabled (using the function view_enabled or enabling them in the Room Editor) then they will not be drawn to the screen.
view_visible[0 ... 7];
Boolean (true: enabled, false: disabled)
view_visible = true;
The above code checks to see if view is visible or not and if it is not, it then sets it to be visible.