External textures drawn at their own size, a repaint without rebuilding the view, an icon size for list rows, and lighter tabs on the desktop strip

External textures. `ExternalSource::Texture` stretches its texture over the widget's rect, so a producer that is a few frames behind a new rect shows its old picture stretched or squeezed. A web page while its window is resized is the clearest case: the page keeps sending frames of the old size until it has laid itself out again, and the content wobbles as the window moves. The new `ExternalSource::Frame` closure returns the texture with its size in physical pixels, and the texture is drawn at that size from the rect's top left corner, cut to the rect. A late frame then leaves an empty edge or loses one for a moment, as other browsers do, instead of being distorted. The cut uses `interior_clip`, which moves from the text field's drawing code to `render/clip.rs` so both widgets can use it, and the clip that was active before is restored afterwards.
Repaint without a view. An application whose only change is a new frame in an external texture had to report `InvalidationScope::All`, which rebuilds the whole view just to draw the same widgets again. `App::needs_repaint` is asked once per iteration, after the messages are processed. When it returns `true`, the main surface and the secondary surfaces are redrawn from the cached view, without calling `view`. The default is `false`, so existing applications behave as before.
List rows. `ListItem::icon_size` overrides the size of the leading icon, which was always the theme's 24 design px, for dense lists such as a menu, where an icon of that size dominates the row.
Tab strip. On the desktop, a tab's title is drawn at the regular weight whether it is selected or not; the selected tab already stands out by its fill and edge, and the bold title made the strip heavy. The desktop tab's close icon is 10 widget pixels instead of the 16 that the cards of the tab overview keep, so `parts::close` now takes the icon size from its caller.

src/app.rs

@@ -1218,6 +1218,9 @@ pub trait App: 'static
 	/// The event loop will keep requesting redraws at ~60 fps until this returns `false`.
 	fn is_animating( &self ) -> bool { false }
 
+	/// Return `true` when the main surface's pixels changed but its widgets did not, such as an [`crate::widget::external::External`] with a new frame: the surface is drawn again from the cached view, without calling [`Self::view`]. Asked once per iteration, after the messages are processed.
+	fn needs_repaint( &mut self ) -> bool { false }
+
 	/// Cap [`Self::is_animating`] redraws to ~30 Hz when the **software**
 	/// renderer is active. Default `true`: 60 fps software rasterization is a
 	/// common battery sink on mobile (no GPU offload), and halving it is