Add utility function for merging directory names with filenames, and fix issue in mj_addfileVFS where directory names without trailing filesystem separators won't append correctly.

PiperOrigin-RevId: 545450158
Change-Id: I550394f11ac2837d35f3d869f8718a119a9eac38

src/engine/engine_util_misc.h

@@ -49,7 +49,7 @@ MJAPI mjtNum mju_muscleDynamics(mjtNum ctrl, mjtNum act, const mjtNum prm[3]);
 // all 3 semi-axes of a geom
 MJAPI void mju_geomSemiAxes(const mjModel* m, int geom_id, mjtNum semiaxes[3]);
 
-//------------------------------ misclellaneous ----------------------------------------------------
+//------------------------------ miscellaneous ----------------------------------------------------
 
 // convert contact force to pyramid representation
 MJAPI void mju_encodePyramid(mjtNum* pyramid, const mjtNum* force,
@@ -130,6 +130,10 @@ MJAPI mjtNum mju_Halton(int index, int base);
 // call strncpy, then set dst[n-1] = 0
 MJAPI char* mju_strncpy(char *dst, const char *src, int n);
 
+// assemble full filename from directory and filename, return 0 on success
+MJAPI int mju_makefullname(char* full, size_t nfull,
+                           const char* dir, const char* file);
+
 // sigmoid function over 0<=x<=1 using quintic polynomial
 MJAPI mjtNum mju_sigmoid(mjtNum x);