feat(render): logical style metrics with a UI scale, so chrome has a consistent physical size

examples/gtk_demo.cpp

@@ -421,9 +421,11 @@ gboolean on_exit_timer(gpointer user_data) {
   const plot::Rect viewport = demo.area->device_viewport();
 
   // The evidence that the layout was *recomputed and not scaled*: the y label
-  // strip is a number of *font pixels*, so it stays the same size while the axes takes
-  // the rest of the new allocation. Two things are checked, and neither is "the
-  // strip is the same number":
+  // strip is measured from the labels' font metrics -- a logical size, converted to
+  // device pixels at *this* display's scale (see `Style::ui_scale`) -- so at a
+  // constant scale it stays the same size while the axes takes the rest of the new
+  // allocation. Two things are checked, and neither is "the strip is the same
+  // number":
   //
   //   * the axes and its strips tile the *current* viewport exactly -- a layout
   //     left over from the old geometry would still tile the old one;