[util] Add private reference count to COM object

This can be used in case DXVK needs to keep a strong reference
to an object but may not expose that reference to the application.

src/util/com/com_object.h

@@ -6,6 +6,22 @@
   
 namespace dxvk {
   
+  /**
+   * \brief Reference-counted COM object
+   *
+   * This can serve as a templated base class for most
+   * COM objects. It implements AddRef and Release from
+   * \c IUnknown, and provides methods to increment and
+   * decrement private references which are not visible
+   * to the application.
+   * 
+   * Having two reference counters is sadly necessary
+   * in order to not break games which steal internal
+   * references if the refefence count of an object is
+   + greater than they expect. DXVK sometimes requires
+   * holding on to objects which the application wants
+   * to delete.
+   */
   template<typename... Base>
   class ComObject : public Base... {
     
@@ -14,21 +30,36 @@ namespace dxvk {
     virtual ~ComObject() { }
     
     ULONG STDMETHODCALLTYPE AddRef() {
-      return ++m_refCount;
+      ULONG refCount = m_refCount++;
+      if (refCount == 0ul)
+        AddRefPrivate();
+      return refCount;
     }
     
     ULONG STDMETHODCALLTYPE Release() {
       ULONG refCount = --m_refCount;
-      if (refCount == 0) {
-        m_refCount += 0x80000000u;
+      if (refCount == 0ul)
+        ReleasePrivate();
+      return refCount;
+    }
+
+
+    void AddRefPrivate() {
+      ++m_refPrivate;
+    }
+
+
+    void ReleasePrivate() {
+      if (--m_refPrivate == 0ul) {
+        m_refPrivate += 0x80000000;
         delete this;
       }
-      return refCount;
     }
     
   private:
     
-    std::atomic<ULONG> m_refCount = { 0ul };
+    std::atomic<ULONG> m_refCount   = { 0ul };
+    std::atomic<ULONG> m_refPrivate = { 0ul };
     
   };