Adds a --overlay-previews flag that switches the hold-Alt tab
overlay from icon+title rows to icon plus a small live view of
each window (three times as tall as a title row, no captions).
Tabs with no mapped window keep their command-line text row.
Thumbnails stay live while the overlay is held: hidden windows
get no scene frame callbacks, so a ~15fps tick paces them with
frame-done (disarmed on release; the plain title list never
wakes background clients). Each commit is routed to its fresh
source -- a CPU snapshot for SHM attaches, a locked dma-buf
import for GPU clients, or the surface's own texture for
in-place SHM updates -- and hidden commits force a repaint.
Also adds a wltest --pulse MS mode (alternating color/black on
frame callbacks) as a deterministic live source, three smoke
tests (appear on open, map mid-hold, pulse flip), and README
docs.
Andrew Hamilton
a9e5f3dcad