Add Flex component.

PiperOrigin-RevId: 572830650
Change-Id: I6908228087b7b9683be3506c8d9cdc725ed5dcd5

src/engine/engine_vis_interact.h

@@ -72,10 +72,11 @@ MJAPI void mjv_applyPerturbForce(const mjModel* m, mjData* d, const mjvPerturb*
 // return the average of two OpenGL cameras
 MJAPI mjvGLCamera mjv_averageCamera(const mjvGLCamera* cam1, const mjvGLCamera* cam2);
 
-// Select geom or skin with mouse, return bodyid; -1: none selected.
+// Select geom, flex or skin with mouse, return bodyid; -1: none selected.
 MJAPI int mjv_select(const mjModel* m, const mjData* d, const mjvOption* vopt,
                      mjtNum aspectratio, mjtNum relx, mjtNum rely,
-                     const mjvScene* scn, mjtNum selpnt[3], int geomid[1], int skinid[1]);
+                     const mjvScene* scn, mjtNum selpnt[3],
+                     int geomid[1], int flexid[1], int skinid[1]);
 
 #ifdef __cplusplus
 }