Pass argc pointer to parse_options()

This lets us keep argc up to date as the backend picks out arguments
from the argv array.
diff --git a/clients/window.h b/clients/window.h
index 646b3dd..c2946d8 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -53,7 +53,7 @@
 };
 
 struct display *
-display_create(int argc, char *argv[]);
+display_create(int *argc, char *argv[]);
 
 void
 display_destroy(struct display *display);