Moved a bunch of definitions from gdi.h into a new gdi_private.h to
make sure we don't use them outside of gdi32.

diff --git a/objects/metafile.c b/objects/metafile.c
index 4e233a3..656bcd6 100644
--- a/objects/metafile.c
+++ b/objects/metafile.c
@@ -54,6 +54,7 @@
 #include "wownt32.h"
 #include "winreg.h"
 #include "winternl.h"
+#include "gdi_private.h"
 #include "wine/debug.h"
 
 WINE_DEFAULT_DEBUG_CHANNEL(metafile);