wined3d: Add a registry key to allow/disallow multisampling.

Right now it is set to disabled due to an Nvidia GLXBadDrawable
bug. Second there is some issue (driver bug too?) in FBO mode.
diff --git a/dlls/wined3d/wined3d_private.h b/dlls/wined3d/wined3d_private.h
index 6f4836e..26952be 100644
--- a/dlls/wined3d/wined3d_private.h
+++ b/dlls/wined3d/wined3d_private.h
@@ -248,6 +248,7 @@
 /* Memory tracking and object counting */
   unsigned int emulated_textureram;
   char *logo;
+  int allow_multisampling;
 } wined3d_settings_t;
 
 extern wined3d_settings_t wined3d_settings;