Add support in mjModel for tagging textures with different roles, and use the slot index of a texture to designate its role.

PiperOrigin-RevId: 654030065
Change-Id: I34a5b132a7f91a6bb8f260500c1b9c128a03b596

python/mujoco/indexer_xmacro.h

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

python/mujoco/usd/exporter.py

@@ -252,7 +252,9 @@ class USDExporter:
     assert geom_name not in self.geom_names
 
     texture_file = (
-        self.texture_files[self.model.mat_texid[geom.matid][0]]
+        self.texture_files[
+            self.model.mat_texid[geom.matid][mujoco.mjTEXROLE_RGB]
+        ]
         if geom.matid != -1
         else None
     )