Add support for multiple textures in one material (mjModel).

PiperOrigin-RevId: 648732764
Change-Id: I29b60dd62a069789a32017ff0d55e36b57e0052b

python/mujoco/indexer_xmacro.h

@@ -168,7 +168,7 @@
   X( float,   light_, specular,      nlight,  3 )
 
 #define MJMODEL_MATERIAL                   \
-  X( int,     mat_, texid,       nmat, 1 ) \
+  X( int,     mat_, texid,       nmat, mjNTEXMAT ) \
   X( mjtByte, mat_, texuniform,  nmat, 1 ) \
   X( float,   mat_, texrepeat,   nmat, 2 ) \
   X( float,   mat_, emission,    nmat, 1 ) \

python/mujoco/usd/exporter.py

@@ -233,7 +233,7 @@ class USDExporter:
     assert geom_name not in self.geom_name2usd
 
     texture_file = (
-        self.texture_files[self.model.mat_texid[geom.matid]]
+        self.texture_files[self.model.mat_texid[mujoco.mjNTEXMAT*geom.matid]]
         if geom.matid != -1
         else None
     )