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/graphics/mapping.c b/graphics/mapping.c
index 43038da..507c789 100644
--- a/graphics/mapping.c
+++ b/graphics/mapping.c
@@ -20,6 +20,7 @@
 
 #include "gdi.h"
 #include "wownt32.h"
+#include "gdi_private.h"
 #include "wine/debug.h"
 
 WINE_DEFAULT_DEBUG_CHANNEL(gdi);