Section: leave the origin and construction planes and axes whole (#1286)

Section Analysis clipped the origin and construction planes and axes along with the model. They now draw from their own unclipped lists, with the same depth and blending; bodies, edges, sketches and highlights are clipped as before. Fixes #1229.

Co-authored-by: DuBearte <[email protected]>

assets/shaders/viewport.wgsl

@@ -200,6 +200,12 @@ fn fs_tri(i: TOut) -> @location(0) vec4<f32> {
     return shade(i);
 }
 
+/// Reference planes: shaded like `fs_tri`, but never cut by the section plane.
+@fragment
+fn fs_tri_all(i: TOut) -> @location(0) vec4<f32> {
+    return shade(i);
+}
+
 fn shade(i: TOut) -> vec4<f32> {
     // A zero normal marks flat colour (selection fills, translucent planes).
     if (length(i.n) < 0.5) {