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_setconst.c

@@ -714,6 +714,7 @@ static void makeFlexBandwidth(mjModel* m, mjData* d) {
     for (int f = 0; f < m->nflex; f++) {
       if (!m->flex_interp[f]) continue;
       int order = m->flex_interp[f];
+      order = order < 0 ? -order : order;
       int nodeadr = m->flex_nodeadr[f];
       int nodenum = m->flex_nodenum[f];
       int cx = m->flex_cellnum[3*f+0];