Use a different magic for memory DCs and get rid of the DC_MEMORY
flag.

diff --git a/objects/bitmap.c b/objects/bitmap.c
index a78d192..e617c81 100644
--- a/objects/bitmap.c
+++ b/objects/bitmap.c
@@ -407,7 +407,7 @@
     DC *dc = DC_GetDCPtr( hdc );
 
     if (!dc) return 0;
-    if (!(dc->flags & DC_MEMORY))
+    if (GetObjectType( hdc ) != OBJ_MEMDC)
     {
         GDI_ReleaseObj( hdc );
         return 0;