Say narrow_rel_base, and let a container pass None for it

`narrow` said what the argument did to a value it never named, so a reader
had to go and find out which value. It is `narrow_rel_base`, and the
resolved one stays the bare `rel_base` -- which is also the only one in
`Painter`, `Placing` and `LayoutHolds`, where there is nothing to tell it
apart from.

It takes `impl Into<Option<[Option<Len>; 2]>>`, so a container that does not
narrow anything writes `None` rather than `[None; 2]`, and `Span` builds the
one case that does with a `then` instead of a mutable array.

Cold layout is byte-identical to `84dad21`.

core/src/ui/render_state.rs

@@ -33,7 +33,7 @@ pub(super) struct DrawInfo {
     pub asked: [Place; 2],
     /// A rel base the parent decided for it on each axis, as a length of the
     /// window, which the widget's own declaration is a fraction of.
-    pub narrow: [Option<Len>; 2],
+    pub narrow_rel_base: [Option<Len>; 2],
     /// Whether the parent already asked about this widget in this draw.
     pub re_asked: bool,
     /// The rel base in pixels, resolved once against the window.
@@ -133,7 +133,7 @@ impl UiRenderState {
         }
     }
 
-    /// The root is asked about in the output. Its own rules narrow both its
+    /// The root is asked about in the output. Its own rules narrow_rel_base both its
     /// rel base and box; nothing above it chose a different one.
     fn root_info(&self, rel_base: UiVec2, region: UiRegion) -> DrawInfo {
         let px = rel_base.to_px(self.output_size);
@@ -148,7 +148,7 @@ impl UiRenderState {
             region,
             placed: [Place::Within(Part::WHOLE); 2],
             asked: [Place::Within(Part::WHOLE); 2],
-            narrow: [None; 2],
+            narrow_rel_base: [None; 2],
             re_asked: false,
             px,
         }
@@ -273,7 +273,7 @@ impl UiRenderState {
         // and what of its own box it asked in. A local redraw asks the same
         // question again from these.
         active.rel_base = info.rel_base;
-        active.narrow = info.narrow;
+        active.narrow_rel_base = info.narrow_rel_base;
         active.re_asked = info.re_asked;
         active.answer = Some(answer);
         active.asked = info.asked;
@@ -470,7 +470,7 @@ impl UiRenderState {
                         region: UiRegion::FULL,
                         placed: [Place::Within(Part::WHOLE); 2],
                         asked: [Place::Within(Part::WHOLE); 2],
-                        narrow: [None; 2],
+                        narrow_rel_base: [None; 2],
                         re_asked: false,
                         px,
                     },
@@ -484,7 +484,7 @@ impl UiRenderState {
             id,
             placement: region,
             rel_base: info.rel_base,
-            narrow: info.narrow,
+            narrow_rel_base: info.narrow_rel_base,
             placed: info.placed,
             asked: info.asked,
             region,
@@ -741,7 +741,7 @@ impl UiRenderState {
             region,
             placed: place,
             asked: active.asked,
-            narrow: active.narrow,
+            narrow_rel_base: active.narrow_rel_base,
             re_asked: active.re_asked,
             px: rel_base.to_px(at.window),
         };
@@ -757,7 +757,7 @@ impl UiRenderState {
             at.region,
             at.rel_base,
             place,
-            active.narrow,
+            active.narrow_rel_base,
             active.declared,
             active.own_align,
         )
@@ -886,7 +886,7 @@ impl UiRenderState {
                 id,
                 placement: UiRegion::FULL,
                 rel_base: UiVec2::FULL_SIZE,
-                narrow: [None; 2],
+                narrow_rel_base: [None; 2],
                 placed: [Place::Within(Part::WHOLE); 2],
                 asked: [Place::Within(Part::WHOLE); 2],
                 region: UiRegion::FULL,
@@ -1146,7 +1146,7 @@ impl UiRenderState {
             region,
             placed: active.asked,
             asked: active.asked,
-            narrow: active.narrow,
+            narrow_rel_base: active.narrow_rel_base,
             re_asked: false,
             px: rel_base.to_px(self.output_size),
         };
@@ -1171,7 +1171,7 @@ impl UiRenderState {
         }
         if active.answer != was_answer || active.holds != was_holds {
             // The parent retains both the answer and the drawing's validity;
-            // even an unchanged size can narrow the range safe for a resize.
+            // even an unchanged size can narrow_rel_base the range safe for a resize.
             #[cfg(feature = "layout-diagnostics")]
             {
                 diag::bump(Counter::SizeChanges);