Use a raw function pointer instead of std::function objects in thread pool.

std::function can have an allocation cost when it's created, and also adds overhead to function calls. Since this API must be used from C, using a raw function pointer is good enough.
PiperOrigin-RevId: 562760324
Change-Id: I2b714a13e07a2b71566d013f21f76c506e817a52

include/mujoco/mjthread.h

@@ -27,7 +27,7 @@ extern "C" {
 // These types are implemented in C++, they're just used as opaque pointers in C
 // to provide type safety for functions.
 struct mjTask_ {
-  char buffer[48];
+  char buffer[24];
 };
 typedef struct mjTask_ mjTask;