Expose a handle for the Python viewer.

This change also requires user scripts to explicitly synchronize changes to physics state to the viewer. The Simulate class was reconfigured so that certain UI events are handled during this sync operation, outside of the render loop on the main thread. These correspond to operations that require access to the full mjModel/mjData.

To support other, more interactive operations (e.g. camera movements), a new mjvSceneState struct is introduced which captures only the portion of the physics state required for scene re-rendering. The mjvSceneState is updated from mjModel/mjData during the viewer sync operation, and is significantly cheaper than a full mj_copyModel and mj_copyData.

Fixes google-deepmind/mujoco#796

PiperOrigin-RevId: 525723636
Change-Id: I57530ea34423b1075ae699ce1c2d988182730ae5

dm_control/autowrap/header_parsing.py

@@ -80,8 +80,9 @@ def _nested_if_else(if_, pred, else_, endif, match_if_true, match_if_false):
 
 # Some common string patterns to suppress.
 # ------------------------------------------------------------------------------
-(X, LPAREN, RPAREN, LBRACK, RBRACK, LBRACE, RBRACE, SEMI, COMMA, EQUAL, FSLASH,
- BSLASH) = list(map(pp.Suppress, "X()[]{};,=/\\"))
+(LPAREN, RPAREN, LBRACK, RBRACK, LBRACE, RBRACE, SEMI, COMMA, EQUAL, FSLASH,
+ BSLASH) = list(map(pp.Suppress, "()[]{};,=/\\"))
+X = (pp.Keyword("X") | pp.Keyword("XMJV")).suppress()
 EOL = pp.LineEnd().suppress()
 
 # Comments, continuation.