osc: add back hover-sec in draw-preview

this was initially part of draw-preview but was later moved to a
separate property because it was deemed useful outside of just
thumbnail preview. however this causes bugs when a thumbnailer
observes draw-request and the draw rectangle doesn't change (e.g
due to getting clamped around the border).

add back hover-sec which makes the draw-request more self
contained and removes this footgun.

Fixes: https://github.com/mpv-player/mpv/pull/17518#discussion_r3256017469