Fix for mermaid rendering when the diagram is initially out of view

[akh00]

Mermaid diagrams and remote URL images render asynchronously — their placement has no .path until the render/download completes. process_one skips any placement not near the viewport, and the WinScrolled retry in place_images only re-processes placements that already have a .path. So an async-render placement below the initial viewport is never processed (skipped at setup, ignored by the scroll retry) and stays stuck on the "Rendering mermaid diagram…" placeholder forever — even after scrolling onto it. Static images with a near-top block render fine, which masks the bug

Repro: any markdown where the ```mermaid block sits below the fold; e.g. :MdRender demo inside the small terminal window

Fix: process mermaid_source/src_url placements regardless of viewport (matching what update_images already does); static local images keep the lazy-viewport optimization since they retry correctly on scroll.