ARB sync: Fix delete behavior and context destruction behavior

I believe this resolves the outstanding issues WRT sync object
deletetion.  I have also added a large comment at the top of syncobj.c
describing the expected memory management behavior.  I'm still a
little uncertain about the locking on ctx->Shared.

src/mesa/main/shared.c

@@ -43,7 +43,9 @@
 #if FEATURE_ATI_fragment_shader
 #include "shader/atifragshader.h"
 #endif
-
+#if FEATURE_ARB_sync
+#include "syncobj.h"
+#endif
 
 /**
  * Allocate and initialize a shared context state structure.
@@ -127,6 +129,10 @@ _mesa_alloc_shared_state(GLcontext *ctx)
    shared->RenderBuffers = _mesa_NewHashTable();
 #endif
 
+#if FEATURE_ARB_sync
+   make_empty_list(& shared->SyncObjects);
+#endif
+
    return shared;
 }
 
@@ -336,6 +342,17 @@ _mesa_free_shared_state(GLcontext *ctx, struct gl_shared_state *shared)
    ctx->Driver.DeleteBuffer(ctx, shared->NullBufferObj);
 #endif
 
+#if FEATURE_ARB_sync
+   {
+      struct simple_node *node;
+      struct simple_node *temp;
+
+      foreach_s(node, temp, & shared->SyncObjects) {
+	 _mesa_unref_sync_object(ctx, (struct gl_sync_object *) node);
+      }
+   }
+#endif
+
    /*
     * Free texture objects (after FBOs since some textures might have
     * been bound to FBOs).