Enable compiler errors for implicit switch case fallthroughs.

Also unify warning options across Clang and GCC and fix minor issues that was surfaced by this.

PiperOrigin-RevId: 508619655
Change-Id: I59b777bf2dfea4422485670e2427c7d3f5cf6405

src/engine/engine_sensor.c

@@ -21,6 +21,7 @@
 #include <mujoco/mjplugin.h>
 #include "engine/engine_callback.h"
 #include "engine/engine_core_smooth.h"
+#include "engine/engine_crossplatform.h"
 #include "engine/engine_io.h"
 #include "engine/engine_macro.h"
 #include "engine/engine_plugin.h"
@@ -804,6 +805,7 @@ void mj_energyPos(const mjModel* m, mjData* d) {
 
         // continue with rotations
         padr += 3;
+        mjFALLTHROUGH;
 
       case mjJNT_BALL:
         // covert quatertion difference into angular "velocity"