Partial stub of GdiAlphaBlend/AlphaBlend.

diff --git a/include/wingdi.h b/include/wingdi.h
index b716864..ce66b8b 100644
--- a/include/wingdi.h
+++ b/include/wingdi.h
@@ -3133,6 +3133,9 @@
     ULONG LowerRight;
 } GRADIENT_RECT, *PGRADIENT_RECT, *LPGRADIENT_RECT;
 
+#define AC_SRC_OVER  0x00
+#define AC_SRC_ALPHA 0x01
+
 typedef struct _BLENDFUNCTION
 {
     BYTE BlendOp;