Cycles OSL: support for the trace(point pos, vector dir, ...) function, to trace

rays from the OSL shader. The "shade" parameter is not supported currently, but
attributes can be retrieved from the object that was hit using the
getmessage("trace", ..) function.

As mentioned in the OSL specification, this function can't be used instead of
lighting, the main purpose is to allow shaders to "probe" nearby geometry, for
example to apply a projected texture that can be blocked by geometry, apply
more “wear” to exposed geometry, or make other ambient occlusion-like effects.

http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Nodes/OSL#Trace

Example .blend and render:
http://www.pasteall.org/blend/17347
http://www.pasteall.org/pic/show.php?id=40066

intern/cycles/kernel/osl/osl_globals.h

@@ -59,6 +59,7 @@ struct OSLGlobals {
 
 	vector<AttributeMap> attribute_map;
 	ObjectNameMap object_name_map;
+	vector<ustring> object_names;
 
 	/* thread key for thread specific data lookup */
 	struct ThreadData {