Add struct cgit_page to cgit_context

This struct is used when generating http headers, and as such is another
small step towards the goal of the whole cleanup series; to invoke each
page/view function with a function pointer.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
diff --git a/ui-snapshot.c b/ui-snapshot.c
index 67dbbdd..4449803 100644
--- a/ui-snapshot.c
+++ b/ui-snapshot.c
@@ -101,7 +101,9 @@
 		args.base = fmt("%s/", prefix);
 		args.tree = commit->tree;
 		args.time = commit->date;
-		cgit_print_snapshot_start(sat->mimetype, filename, item);
+		ctx.page.mimetype = xstrdup(sat->mimetype);
+		ctx.page.filename = xstrdup(filename);
+		cgit_print_http_headers(&ctx);
 		(*sat->write_func)(&args);
 		return;
 	}