Release _pixmap for ThumbnailWidgets outside the visible viewport
plus a 5-row buffer zone. Re-decode from the on-disk thumbnail
cache (_source_path) when they scroll back into view. Caps decoded
thumbnail memory to the visible area instead of growing unboundedly
during infinite scroll.
behavior change: off-screen thumbnails release their decoded
pixmaps and re-decode on scroll-back. No visual difference —
the buffer zone prevents flicker.
pax
b00f3ff95c