[WIN32SS] Enable multidisplay driver when required

For now, it is only a stub which returns an error.

win32ss/CMakeLists.txt

@@ -77,6 +77,7 @@ list(APPEND SOURCE
     gdi/eng/mem.c
     gdi/eng/engmisc.c
     gdi/eng/mouse.c
+    gdi/eng/multidisp.c
     gdi/eng/paint.c
     gdi/eng/pathobj.c
     gdi/eng/pdevobj.c

win32ss/gdi/eng/multidisp.c

@@ -0,0 +1,24 @@
+/*
+* PROJECT:     ReactOS Win32k subsystem
+* LICENSE:     GPL-2.0-or-later (https://spdx.org/licenses/GPL-2.0-or-later)
+* FILE:        win32ss/gdi/eng/multidisp.c
+* PURPOSE:     Multi-Display driver
+* PROGRAMMERS:
+*/
+
+/* INCLUDES *******************************************************************/
+
+#include <win32k.h>
+#define NDEBUG
+#include <debug.h>
+
+BOOL
+APIENTRY
+MultiEnableDriver(
+    _In_ ULONG iEngineVersion,
+    _In_ ULONG cj,
+    _Inout_bytecount_(cj) PDRVENABLEDATA pded)
+{
+    UNIMPLEMENTED;
+    return FALSE;
+}

win32ss/gdi/eng/pdevobj.c

@@ -14,6 +14,13 @@ DBG_DEFAULT_CHANNEL(EngPDev);
 static PPDEVOBJ gppdevList = NULL;
 static HSEMAPHORE ghsemPDEV;
 
+BOOL
+APIENTRY
+MultiEnableDriver(
+    _In_ ULONG iEngineVersion,
+    _In_ ULONG cj,
+    _Inout_bytecount_(cj) PDRVENABLEDATA pded);
+
 CODE_SEG("INIT")
 NTSTATUS
 NTAPI
@@ -456,6 +463,9 @@ PDEVOBJ_Create(
     }
 
     /* Try to get a display driver */
+    if (ldevtype == LDEV_DEVICE_META)
+        pldev = LDEVOBJ_pLoadInternal(MultiEnableDriver, ldevtype);
+    else
         pldev = LDEVOBJ_pLoadDriver(pdm->dmDeviceName, ldevtype);
     if (!pldev)
     {
@@ -877,11 +887,15 @@ PDEVOBJ_lChangeDisplaySettings(
         }
         else
         {
-            /* FIXME: currently, only use the first display */
+            /* Enable MultiDriver */
-            UNIMPLEMENTED;
+            pmdev->ppdevGlobal = PDEVOBJ_Create(NULL, (PDEVMODEW)pmdev, 0, LDEV_DEVICE_META);
+            if (!pmdev->ppdevGlobal)
+            {
+                WARN("Failed to create meta-device. Using only first display\n");
                 PDEVOBJ_vReference(pmdev->dev[0].ppdev);
                 pmdev->ppdevGlobal = pmdev->dev[0].ppdev;
             }
+        }
 
         if (pmdevOld)
         {