Fix monitor disappearing on OS X with AGS

When automatic graphics switching is activated, the CGDirectDisplayID
for the display changes, casuing the primary GLFWmonitor to disappear.

src/cocoa_monitor.m

@@ -268,7 +268,8 @@ _GLFWmonitor** _glfwPlatformGetMonitors(int* count)
         char* name = getDisplayName(displays[i]);
 
         monitor = _glfwAllocMonitor(name, size.width, size.height);
-        monitor->ns.displayID = displays[i];
+        monitor->ns.displayID  = displays[i];
+        monitor->ns.unitNumber = CGDisplayUnitNumber(displays[i]);
 
         free(name);
 
@@ -284,7 +285,9 @@ _GLFWmonitor** _glfwPlatformGetMonitors(int* count)
 
 GLboolean _glfwPlatformIsSameMonitor(_GLFWmonitor* first, _GLFWmonitor* second)
 {
-    return first->ns.displayID == second->ns.displayID;
+    // HACK: Compare unit numbers instead of display IDs to work around display
+    //       replacement on machines with automatic graphics switching
+    return first->ns.unitNumber == second->ns.unitNumber;
 }
 
 void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)

src/cocoa_platform.h

@@ -90,6 +90,7 @@ typedef struct _GLFWmonitorNS
 {
     CGDirectDisplayID   displayID;
     CGDisplayModeRef    previousMode;
+    uint32_t            unitNumber;
 
 } _GLFWmonitorNS;