add support for ppm images

src/att_lock.h

@@ -152,6 +152,11 @@ int  pattern_to_string(const struct att_pattern *p, char *buf, size_t n);
 void render_output(struct att_output *o);
 void render_all(struct att *app);
 
+/* image.c ------------------------------------------------------------------ */
+/* PNG or PPM (P3/P6), by path or "-" for stdin. Returns a cairo_surface_t*,
+ * or NULL after reporting why -- a missing background is never fatal. */
+void *image_load(const char *path);
+
 /* auth.c ------------------------------------------------------------------- */
 enum auth_hash_state {
 	AUTH_HASH_NONE,   /* no stored pattern yet                            */