Answers the review of bdbbc3d7 and the IC8 pass after it.
A shape hidden from Draw's list was only set to alpha 0, so the line's
flowing stroke, which keeps its own colour, stayed painted. Now:
- the world renderer's sync() sets every entity of a hidden mark to
show = false, and FlowMaterialProperty follows the mark's alpha;
- the screen renderer does not project a hidden callout, so it is not drawn
and cannot push a visible label aside;
- hiding acts at once, even before a mark's first frame, and with every
mark hidden the annotations render hold is released after one last frame;
- fly_route skips a hidden route.
Hand-drawn shapes are never merged as duplicates: every one is drawn on
purpose and unnamed until its row names it, so a second pin dropped near
the first was dropped, or replaced the first when its colour differed. A
restyle the renderer cannot draw now also syncs the renderer.
Tests cover each: renderer show/hide and flow alpha, the screen skip, the
render hold and its last frame, a show with no fade-in, hidden routes, two
nearby pins, and the restyle rollback.
