3D and Materials: Inflate takes Twist, Taper and Cap

The public 3D and Materials panel options list Depth, Twist, Taper and Cap
for every 3D type; Inflate had only Depth.

- Twist (0° to 360°) and Taper (1–100 %) turn and scale the back about the
  shape's centre, as Extrude & Bevel's do: a puffed back is turned and
  lowered in proportion, and the sides follow in slices.
- Cap: Solid closes the back; Hollow leaves it off, so the inflated front is
  a shell open behind. The documentation doesn't say what Cap means for
  Inflate; this reading is stated in docs/three-d.md.
- The dialog's Object tab shows them in the documented order (Depth, Twist,
  Taper, Cap), then Volume and Both sides.

At 0 % volume a twisted, tapered Inflate has the extrusion's outline from
every side (tested).

A twisted, tapered, hollow Inflate draws on the canvas exactly as it
exports, and expands to faces with their own ids (tested). ROADMAP and the
parity pages list the new options; the 3D score stays at ~25 %.

docs/three-d.md

@@ -44,6 +44,9 @@ middle.
 | Option | What it does |
 |---|---|
 | Depth | 0–2,000 pt of flat side between the front and the back. |
+| Twist | 0° to 360°: the back turned against the front (its puff too), the sides following in slices. |
+| Taper | 1–100 %: the back's size against the front's; a puffed back is lower in proportion. |
+| Cap | **Solid** closes the back; **Hollow** leaves it off, so the inflated front is a shell open behind, its inside showing. The public documentation lists Cap for every 3D type without saying what it means for Inflate; this is our reading. |
 | Volume | 0–100 %. The surface is a membrane under even pressure, held along the outline (and around holes): it rises vertically from the edge and rounds smoothly inside, without creases along the shape's middle. At 50 % a circle becomes a half sphere; 100 % puffs twice as high, 0 % is flat. |
 | Both sides | The back puffs out too; otherwise it is flat. |