Monitor callback only triggered by physical device events

[elmindreda]

This issue has been moved from SourceForge.

Tested under Windows 8, but probably exists for prior versions, _glfw.monitorCallback is only being called on physical device connects/disconnects. The cause of this is that it relies on the WM_DEVICECHANGE/DBT_DEVNODES_CHANGED message which is, of course, only sent when a physical device is connected or disconnected from the system. This means that enabling or disabling displays (e.g., turning extended display settings on/off) does not trigger a callback and cannot be re-enumerated without restarting the program.

By using Spy++ to watch notifications sent when displays were enabled/disabled I was able to identify a few notifications that are triggered through this action, of which WM_DISPLAYCHANGE is the most promising (the others tended to get sent multiple times). As a proof of concept, I modified windowProc in win32_window.c, but I have concerns about it's suitability as a permanent solution. Off the top of my head, though, I see two reasons why this solution might not be a good one:

• it means that display connects/disconnects will result in _glfwInputMonitorChange being called twice as a result of handling both 'WM_DEVICECHANGEandWM_DISPLAYCHANGE`.
• it means that initiating a video mode change will result in _glfwInputMonitorChange being called unnecessarily.

There is probably a better way of handling this scenario, but Windows isn't my thing and I don't really know how else to integrate support for handling display enabling/disabling with the existing methods for handling notifications.

Regardless, as enabling/disabling monitors without disconnecting them is a common usage scenario this is something that probably should be supported.

[elmindreda]

This is related to #288.