GDI: fix compilation of the render patch on older systems

[Mike McCormack]

This patch builds on Huw's AlphaBlend patch.

ChangeLog:
* fix compilation of the AlphaBlend patch on older systems
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--- dlls/x11drv/xrender.c.old	2004-08-03 00:22:55.000000000 +0900
+++ dlls/x11drv/xrender.c	2004-08-03 00:23:01.000000000 +0900
@@ -47,6 +47,14 @@
 #include <X11/Xlib.h>
 #include <X11/extensions/Xrender.h>
 
+/* Older version of the Xrender headers don't define these */
+#ifndef PictStandardARGB32
+
+#define PictStandardARGB32 0
+XRenderPictFormat * XRenderFindStandardFormat (Display *dpy, int format);
+
+#endif
+
 static XRenderPictFormat *screen_format; /* format of screen */
 static XRenderPictFormat *mono_format; /* format of mono bitmap */