Move meshdir and texturedir to mjsCompiler.

For backwards compatibility in the Python bindings, both `spec.compiler.meshdir` and `spec.meshdir` are now allowed.

Fixes #2834.

PiperOrigin-RevId: 809060113
Change-Id: Ie23f1c5dd57de43568312a8e062906f4013bb474

include/mujoco/mjspec.h

@@ -151,6 +151,8 @@ typedef struct mjsCompiler_ {      // compiler options
   mjtByte saveinertial;            // save explicit inertial clause for all bodies to XML
   int alignfree;                   // align free joints with inertial frame
   mjLROpt LRopt;                   // options for lengthrange computation
+  mjString* meshdir;               // mesh and hfield directory
+  mjString* texturedir;            // texture directory
 } mjsCompiler;
 
 
@@ -161,8 +163,6 @@ typedef struct mjSpec_ {           // model specification
   // compiler data
   mjsCompiler compiler;            // compiler options
   mjtByte strippath;               // automatically strip paths from mesh files
-  mjString* meshdir;               // mesh and hfield directory
-  mjString* texturedir;            // texture directory
 
   // engine data
   mjOption option;                 // physics options