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.

README.md

@@ -88,6 +88,9 @@ GLFW bundles a number of dependencies in the `deps/` directory.
  - [Cocoa] Bugfix: Cursor creation failed unless a window had been created.
  - [Cocoa] Bugfix: Window refresh events were not generated by iconification or
                    restoration
+ - [Cocoa] Bugfix: The primary monitor would get reported as disconnected when
+                   entering full screen on a dual-GPU machine with automatic
+                   graphics switching
  - [X11] Bugfix: `glfwInit` would segfault on systems without RandR
  - [X11] Bugfix: The response to `_NET_WM_PING` was sent to the wrong window
  - [X11] Bugfix: Character input via XIM did not work in many cases

src/cocoa_monitor.m

@@ -269,6 +269,7 @@ _GLFWmonitor** _glfwPlatformGetMonitors(int* count)
 
         monitor = _glfwAllocMonitor(name, size.width, size.height);
         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;