Keep the Surfaces the Roles Name in the Shell-Only Drag Mode

The shell test asks whether at least half of the cells a segment crosses lie on
the footprint's boundary. On a curved wall the boundary is a staircase of cells,
and an outer-wall segment that runs past an inside corner of that staircase
crosses cells whose four neighbours are all filled, so a short segment there
failed the test and vanished, leaving a hole with the inner wall showing through
it. A shallow dome does the same to its top skin: one layer's ring lies within
the same cells as the next layer's, so it is neither a boundary nor the topmost
occupant, and the bridges under it showed through.

Outer walls and the top and bottom skins are the surface by definition, so the
mode now keeps them whatever the grid says, and the grid decides only for the
roles that can be either: inner walls, the prime tower, supports, skirt and brim.

src/libvgcode/src/ViewerImpl.cpp

@@ -1200,6 +1200,20 @@ void ViewerImpl::load(GCodeInputData&& gcode_data)
 }
 
 #ifndef ENABLE_OPENGL_ES
+static bool is_outer_wall(EGCodeExtrusionRole role)
+{
+    return role == EGCodeExtrusionRole::ExternalPerimeter || role == EGCodeExtrusionRole::OverhangPerimeter;
+}
+
+// what is a visible surface by definition, whatever the grid says: the outer walls and the top
+// and bottom skins. A step between one layer and the next is often narrower than a cell, so the
+// grid alone would drop the odd segment of them on a curve and show what lies behind
+static bool is_surface_by_role(EGCodeExtrusionRole role)
+{
+    return is_outer_wall(role) || role == EGCodeExtrusionRole::TopSolidInfill || role == EGCodeExtrusionRole::BottomSurface ||
+           role == EGCodeExtrusionRole::BridgeInfill || role == EGCodeExtrusionRole::Ironing;
+}
+
 // what can never be a visible surface whatever the geometry says: short infill segments hugging
 // a wall would otherwise pass the geometric test by the thousand
 static bool is_hidden_in_shell(EGCodeExtrusionRole role)
@@ -1214,13 +1228,13 @@ bool ViewerImpl::reduced_set_keeps(size_t i, const PathVertex& v) const
 {
     switch (m_settings.reduced_detail_mode) {
     case EReducedDetailMode::OuterWallsOnly:
-        return v.role == EGCodeExtrusionRole::ExternalPerimeter || v.role == EGCodeExtrusionRole::OverhangPerimeter;
+        return is_outer_wall(v.role);
     case EReducedDetailMode::ShellOnly:
         // the first inner wall fills the step of a sloped surface between one layer's outer wall
         // and the next, too narrow for the grid to see; whatever is the visible top or bottom of a
         // step stays whatever its role
-        return (!is_hidden_in_shell(v.role) && m_shell_bitset[i]) || m_near_shell_bitset[i] ||
-               m_top_visible_bitset[i] || m_bottom_visible_bitset[i];
+        return is_surface_by_role(v.role) || (!is_hidden_in_shell(v.role) && m_shell_bitset[i]) ||
+               m_near_shell_bitset[i] || m_top_visible_bitset[i] || m_bottom_visible_bitset[i];
     default:
         return true;
     }
@@ -1571,7 +1585,7 @@ void ViewerImpl::update_shell_bitset()
             outer_walls_by_cell.clear();
             for (size_t i = first; i < last; ++i) {
                 const EGCodeExtrusionRole role = m_vertices[i].role;
-                if (is_drawn_extrusion(i) && (role == EGCodeExtrusionRole::ExternalPerimeter || role == EGCodeExtrusionRole::OverhangPerimeter))
+                if (is_drawn_extrusion(i) && is_outer_wall(role))
                     for_each_cell(i, [&](int x, int y) { outer_walls_by_cell[cell_index(x, y)].push_back(static_cast<uint32_t>(i)); });
             }
             // an inner wall segment is the first inner wall when its midpoint lies within a line