Add elasticity parameters to the schema.

PiperOrigin-RevId: 675115200
Change-Id: Ic6608ce61cbb857dca71b3d9cbdac057e8f1ac7a

src/user/user_init.c

@@ -236,6 +236,7 @@ void mjs_defaultFlex(mjsFlex* flex) {
   flex->activelayers = 1;
   flex->rgba[0] = flex->rgba[1] = flex->rgba[2] = 0.5f;
   flex->rgba[3] = 1.0f;
+  flex->thickness = -1;
 }