Clean-up flex assumptions.

Do not allow a mix of `elastic2d != none` with `dof = trilinear` since the latter assumes 3d elasticity.

Also, do not assume that `flex_interp > 0` in the engine. This will enable to use, e.g., `flex_interp = -1` to mean a linear surface finite element instead of a 3d finite element which is currently identified with `flex_interp = 1`.

PiperOrigin-RevId: 903852035
Change-Id: Ia6290b4a05e9e510ffb7f36d141cd525b40d3110

src/engine/engine_vis_interact.c

@@ -864,6 +864,7 @@ int mjv_select(const mjModel* m, const mjData* d, const mjvOption* vopt,
         if (m->flex_interp[i]) {
           mjtNum* coord = m->flex_vert0 + 3*(m->flex_vertadr[i] + vertid);
           int order = m->flex_interp[i];
+          order = order < 0 ? -order : order;
           int npc = (order+1)*(order+1)*(order+1);
 
           // cell lookup: get local coords and node indices