Fixes #705 by returning a dummy MonitorHandle when no monitors are available. This seemed like the most straightforward fix, as the platform-independent facade would not permit returning an Option<MonitorHandle> from Window::current_monitor. The dummy MonitorHandle has an id value of 0 (an invalid XID value) and a few checks are added to avoid caching this dummy handle or using it as though it were valid.

I was unable to reproduce the original issue myself, so I hope that someone still experiencing the issue will test this and let me know if any new bugs occur as a result of this change.

Overall, what would happen if a user unknowingly tries to use this dummy monitor handle as it were the main monitor for their app, by passing it to set_fullscreen, or by calling the methods of MonitorHandle to get its name / position / dpi factor?

@vberger There's an explicit check for the dummy monitor's ID value in Window::set_fullscreen. If the user requests fullscreen on the dummy monitor, no action is taken.

These are the dummy monitor's fields:

The size of the monitor is zero. I suppose this may cause an application to divide by zero if, e.g., trying to center the window on the current monitor. I could instead give the dummy monitor a size of (1, 1).

cc @jarredbarber (of #1007) and @chutz (of #705). Does this branch fix the issues you reported?

Replaced a return Err with a default DPI factor of 1.0 to avoid another panic. This should address #1007.

Should be good to merge when CI is done?

@zegentzy Yes, I think so.