Make mjs_findElement find textures by file name.

This is done if the model is uncompiled and their name is empty.

PiperOrigin-RevId: 772885774
Change-Id: I827d5b679637fa0571cfb0a69833ed688df16304

src/user/user_model.h

@@ -248,6 +248,7 @@ class mjCModel : public mjCModel_, private mjSpec {
   mjCDef* FindDefault(std::string name);                            // find defaults class name
   mjCDef* AddDefault(std::string name, mjCDef* parent = nullptr);   // add defaults class to array
   mjCBase* FindObject(mjtObj type, std::string name) const;         // find object given type and name
+  mjCBase* FindTexture(std::string_view name) const;                // find texture given name
   mjCBase* FindTree(mjCBody* body, mjtObj type, std::string name);  // find tree object given name
   mjSpec* FindSpec(std::string name) const;                         // find spec given name
   mjSpec* FindSpec(const mjsCompiler* compiler_);                   // find spec given mjsCompiler