Replace strncpy with memcpy for gridlayout initialization.

memcpy is more appropriate here as the source and destination buffers have the same size, ensuring the entire buffer is copied.

PiperOrigin-RevId: 927395770
Change-Id: Idbe9dade3160f593060fae5783743b28c0452695

src/user/user_init.c

@@ -272,7 +272,7 @@ void mjs_defaultTexture(mjsTexture* texture) {
   texture->gridsize[0] = texture->gridsize[1] = 1;
   texture->nchannel = 3;
   char defaultlayout[sizeof(texture->gridlayout)] = "............";
-  strncpy(texture->gridlayout, defaultlayout, sizeof(texture->gridlayout));
+  memcpy(texture->gridlayout, defaultlayout, sizeof(texture->gridlayout));
 }