Add collisions between flexes and SDF geoms.

The flex-SDF algorithm uses the same logic as mesh-SDF. Previous to this change, the flex was instead colliding with the convex hull of the mesh generated automatically with marching cubes from the SDF.

PiperOrigin-RevId: 876420302
Change-Id: I3bf6552b6b8e108ba4c4209bbcf10fe5f2b2fd90

src/engine/engine_collision_sdf.h

@@ -44,6 +44,10 @@ int mjc_MeshSDF(const mjModel* m, const mjData* d, mjContact* con, int g1, int g
 // collision between two signed distance fields
 int mjc_SDF(const mjModel* m, const mjData* d, mjContact* con, int g1, int g2, mjtNum margin);
 
+// collision between entire flex and SDF geom (batched processing)
+int mjc_FlexSDF(const mjModel* m, mjData* d, mjContact* con,
+                int g, int f, mjtNum margin);
+
 #ifdef __cplusplus
 }
 #endif