render: slices, surface values and streamlines of cell flow fields

A fluid step's result is a field per cell of a regular grid, not per
node, so the contour machinery had nothing to draw it with and the
viewport showed nothing after a fluid solve.

Add `render::flow`, CPU display data for such fields with no solver
dependency (render stays at L1): an axis-aligned slice through the
cells as triangles carrying cell-centred values (interpolated between
the two layers either side, outer centres moved to the domain faces,
cells inside obstacles left out), the field sampled at a surface
beside the fluid cells for colouring an obstacle, and streamlines
traced through the trilinear velocity field by the midpoint rule,
seeded on a lattice upstream of the obstacles. Seeds, steps and the
total number of points are capped so hostile counts cost a bounded
time. `contour::banded` fans band pieces into scene triangles for the
callers.

Part of #193.

crates/render/src/lib.rs

@@ -1,7 +1,8 @@
 //! Display data for FieldCraft, with no GPU dependency: what a finite element model looks like
 //! on screen ([`DisplayMesh`]: the visible faces, element edges, line elements and nodes), an
 //! orbit camera with perspective or orthographic projection, picking ([`pick`]: what a ray through
-//! a pixel hits), and a CPU rasterizer for headless snapshots. The desktop viewport draws the
+//! a pixel hits), flow fields on a grid of cells ([`flow`]: slices, surface values and
+//! streamlines), and a CPU rasterizer for headless snapshots. The desktop viewport draws the
 //! same [`Scene`] on the GPU.
 #![deny(clippy::unwrap_used, clippy::expect_used, clippy::panic, clippy::unimplemented, clippy::todo, clippy::unreachable)]
 #![forbid(unsafe_code)]
@@ -9,6 +10,7 @@
 mod camera;
 pub mod contour;
 mod display;
+pub mod flow;
 mod font;
 pub mod glyphs;
 pub mod pick;