Design: vertex picking — a click near a corner takes the corner

Completes the precedence Tommaso asked for: vertex, then edge, then face, all
from ONE click, all decided in screen pixels. Verified on :11 by sweeping into
the right corner of a plate — x=1250 and 1290 report "face 5 selected", x=1308,
1318 and 1323 report "vertex selected" — and the cyan marker lands exactly on
the corner in the render.

The vertex tolerance (11 px) is deliberately LARGER than the edge one (8 px). A
corner lies ON its edges, so equal radii would make vertices unreachable: every
click near one would resolve to the edge underneath. Bigger-wins-first is what
makes the smallest entity actually pickable.

Vertices come from the sampled edge polylines' endpoints rather than a separate
topology walk — every corner of a face is the end of one of its edges, so the
data was already in hand.

The highlight is a camera-facing square scaled by 1/zoom, the same trick the
edge ribbon uses, so it reads as a constant dot at any zoom. This is why vertex
picking did not ship with the previous commit: the Edge render path billboards
a ribbon and degenerates on a two-point input, and a selection you cannot see
is not a selection (L5). Better to add the primitive than to fake the feature.

DesignPanel now distinguishes level 4: a picked corner sets neither face nor
edge, because a corner is not its face, and the offer classifies it as
OfferSel::Vertex — where Plane, Axis and Coord Sys already accept it.

snaporca-9xw (rubber band still open — see the issue).

src/slic3r/GUI/DesignPanel.hpp

@@ -593,6 +593,7 @@ private:
     int               m_sel_solid_body{-1};   // which body the face/edge selection is on
     int               m_sel_solid_face{-1};
     int               m_sel_solid_edge{-1};
+    bool              m_sel_solid_vertex{false};   // a corner is picked (body+point, no face/edge)
     // The face actually under the last solid click, INDEPENDENT of the whole/face/edge cycle level.
     // The first click on a solid selects the WHOLE body, but the ray has already resolved which face
     // it hit and the callback passes it. "Sketch on the face I clicked" must not require discovering