Make alignment a widget property

tests/retained.rs

@@ -159,10 +159,15 @@ fn a_span_child_that_declares_its_length_is_drawn_once() {
     h.set_root((hinted, asked).span(Dir::RIGHT));
 
     assert_eq!(told_draws.get(), 1);
+    // Reading its box makes its drawing hold for the measuring box alone,
+    // and it reports less than that box: so it is drawn again in the box its
+    // answer places it in, and once more in the final box the span chooses.
+    // A widget that says what it holds for, as text does, skips the middle
+    // one.
     assert_eq!(
         asked_draws.get(),
-        2,
-        "drawn to be measured, then again in its final box"
+        3,
+        "drawn to be measured, in its placed box, then in its final box"
     );
 }
 
@@ -256,6 +261,11 @@ impl Widget for ReadsBox {
 
 /// Reads its box across one axis only, so its drawing holds for a taller
 /// box on its own and only a wider one is worth a draw.
+///
+/// Both of these report a quarter of what they read, without saying that the
+/// drawing holds there too, so each length they are asked at costs two draws:
+/// one to answer, and one in the quarter-sized box that answer places them
+/// in. The counts below are in those pairs.
 struct ReadsWidth {
     draws: Rc<Cell<usize>>,
 }
@@ -336,7 +346,7 @@ fn a_resize_redraws_what_read_its_box() {
     h.resize((800, 100));
     h.frame();
 
-    assert_eq!(draws.get(), settled + 1);
+    assert_eq!(draws.get(), settled + 2);
 }
 
 #[test]
@@ -356,7 +366,7 @@ fn a_resize_only_redraws_read_axes() {
 
     h.resize((800, 300));
     h.frame();
-    assert_eq!(draws.get(), settled + 1, "width changes its answer");
+    assert_eq!(draws.get(), settled + 2, "width changes its answer");
 }
 
 #[test]
@@ -378,7 +388,7 @@ fn subpixel_resize_changes_accumulate_from_the_last_layout() {
 
     h.resize((400.06, 200.0));
     h.frame();
-    assert_eq!(draws.get(), settled + 1);
+    assert_eq!(draws.get(), settled + 2);
 }
 
 #[test]