Fixed monitor size using wrong incorrect value.

The display device string was used, instead of the adapter device name,
leading to the wrong incorrect physical size being returned for
non-primary monitors.  The right incorrect physical size is now returned
for all monitors.
Enabled enumeration of mirroring displays.

This exposes all monitors in every mirroring group, which is needed for
robust detection of an Oculus Rift.
Expose display device name as UTF-8.

This makes interaction with libOVR simpler, as it provides the display
device name in the same encoding.
Added glfwGetMirroredMonitor.