⚡ Render eligible frame drop shadows without filter surfaces

[superalex]

What

• Large clipped frames with drop shadows were expensive to pan/zoom because each shadow went through filter surfaces, per-child silhouettes, and a full-surface blur saveLayer.
• Eligible frames now render the drop shadow from container geometry only, with a tightly bounded blur layer.

Why

• Frame drop shadows at high zoom were dominated by Gaussian blur over the whole DropShadows surface plus descendant silhouette work that does not change the visible result when children stay inside a filled, axis-aligned frame.
• Clipped frames already hide overflow, so walking every descendant extrect on each tile was extra CPU for the common case.

How

• Fast path: filled, axis-aligned frames without layer/background blur use render_frame_container_drop_shadow (rrect + blur saveLayer on DropShadows) instead of filter surfaces and child silhouettes.
• Blur bounds: the blur saveLayer uses compute_fast_bounds so Skia does not allocate the offscreen layer to the full surface clip.
• Clipped frames: skip the descendant extrect walk; still reject the fast path when a descendant has its own drop shadow.
• Overflow frames: still require descendants to be contained in the frame selrect.
• Strokes: the fast path shadows fill geometry only, so outer/center strokes can look slightly narrower than the slow path; those frames stay eligible for performance.