Present visible tiles via ViewportReady so zoom settle turns sharp
without waiting on the interest ring, and paint at atlas slot size so
DPR 2 does not rasterize 1024 only to downscale into 512 slots.
Add a direct container-geometry path for eligible frames: inline blur
when the kernel fits the tile margin, otherwise a cached filter-surface
pass reused across tiles via DropShadowFilterCache on both the direct
and slow render_shape paths.
Move frame shadow logic into shadows.rs. Fix nested/clipped frame
shadows by deferring parent clip to composite time, apply negative
spread via inset, and allow rotated/transformed frames on the direct
path. Skip descendant extrect walks for clipped frames when only
nested drop shadows matter, and skip child silhouettes when the
container fill already covers shadow descendants.
Avoid an offscreen buffer per Fill::Image during tile walks: only use
save_layer when a shape image filter is present; axis-aligned rects and
frames without corner radii also skip the redundant container clip.
