app: add App::subsurface_motion_only so a slide-to-reveal panel animates without re-rastering the main surface

An app whose gesture/animation only moves an input-transparent subsurface (a slide-to-reveal panel over a static main surface) still paid a full main-surface re-raster on every frame of the slide, because the runtime force-dirties the main view on two paths it cannot tell apart from a real content change: `MoveOutcome::Swipe` calls `dirty_caches()` + `request_redraw()` per motion sample, and the `WlCallback` Main handler sets `view_dirty` + `request_redraw()` on every frame callback while `is_animating`. The subsurface reconcile already repositions the panel with a cheap `set_position` + bare parent commit, so the main re-raster is wasted work — and on slow targets (Librem5) it competes with the reposition and makes the slide stutter.
New opt-in `App::subsurface_motion_only` (default `false`, so existing apps are unaffected). When it returns `true`:
- the swipe dispatch (`input/dispatch/outcomes.rs`) still calls the `on_swipe_progress` family but skips `dirty_caches()` / `request_redraw()`; incoming motion events pump the loop and the per-iteration `reconcile_subsurfaces` carries the move.
- the frame-callback animation pump (`event_loop/handlers.rs`) keeps the vsync cadence without a re-raster by requesting a bare `wl.frame()` + `commit()` on the main surface (no buffer attach) instead of dirtying the view; `poll_external` advances the animation and the reconcile repositions each frame.
The main surface still redraws for genuine content changes (messages, resize, the one-shot redraw on swipe release) — only the per-frame slide raster is dropped.

src/input/dispatch/outcomes.rs

@@ -50,12 +50,18 @@ impl<A: App> AppData<A>
 				if let Some( v ) = down       { self.app.on_swipe_down_progress( v ); }
 				if let Some( v ) = horizontal { self.app.on_swipe_horizontal_progress( v ); }
 				// Swipe-progress callbacks mutate app state outside
-				// `update`, so the cached view tree is stale.
-				self.dirty_caches();
-				self.surface_mut( focus ).request_redraw();
-				for ss in self.overlays.values_mut()
+				// `update`, so the cached view tree is stale — unless the
+				// motion only slides a subsurface over a static main
+				// surface, in which case the per-frame subsurface reconcile
+				// carries the move and a main re-raster is wasted work.
+				if !self.app.subsurface_motion_only()
 				{
-					ss.request_redraw();
+					self.dirty_caches();
+					self.surface_mut( focus ).request_redraw();
+					for ss in self.overlays.values_mut()
+					{
+						ss.request_redraw();
+					}
 				}
 			}
 		}