icon-loader: load SVG icons at max scale of any usable output

- Add a new function to get the maximum scale of all usable outputs
- Pass the maximum output scale through to img_svg_load(), which
  ultimately calls cairo_surface_set_device_scale() before rendering

include/icon-loader.h

@@ -7,6 +7,6 @@ struct server;
 void icon_loader_init(struct server *server);
 void icon_loader_finish(struct server *server);
 struct lab_data_buffer *icon_loader_lookup(struct server *server,
-	const char *app_id, int size, int scale);
+	const char *app_id, int size, float scale);
 
 #endif /* LABWC_ICON_LOADER_H */

include/img/img-svg.h

@@ -5,6 +5,6 @@
 struct lab_data_buffer;
 
 void img_svg_load(const char *filename, struct lab_data_buffer **buffer,
-	int size);
+	int size, float scale);
 
 #endif /* LABWC_IMG_SVG_H */

include/labwc.h

@@ -546,6 +546,14 @@ struct wlr_box output_usable_area_scaled(struct output *output);
 void handle_output_power_manager_set_mode(struct wl_listener *listener,
 	void *data);
 void output_enable_adaptive_sync(struct wlr_output *output, bool enabled);
+
+/**
+ * output_max_scale() - get maximum scale factor of all usable outputs.
+ * Used when loading/rendering resources (e.g. icons) that may be
+ * displayed on any output.
+ */
+float output_max_scale(struct server *server);
+
 void new_tearing_hint(struct wl_listener *listener, void *data);
 
 void server_init(struct server *server);