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/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 */