Fix mju_getfiledir when the path contains a mixture of / and \.

PiperOrigin-RevId: 529754511
Change-Id: Ifce2b86bc339a8cae584db639b7901b270e56384

src/engine/engine_resource.h

@@ -37,8 +37,8 @@ MJAPI int mju_readResource(mjResource* resource, const void** buffer);
 // sets for a resource with a name partitioned as {dir}{filename}, the dir and ndir pointers
 MJAPI void mju_getResourceDir(mjResource* resource, const char** dir, int* ndir);
 
-// get directory of a file in the OS filesystem
-void mju_getfiledir(const char* filename, const char** dir, int* ndir);
+// get the length of the dirname portion of a given path
+int mju_dirnamelen(const char* path);
 
 // read file into memory buffer (allocated here with mju_malloc)
 void* mju_fileToMemory(const char* filename, int* filesize);