2.1.3

This is a minor release, handling issues discovered after the launch of 2.1.2.

General

1. simulate now support cycling through cameras (with [ and ] keys).
2. mjVIS_STATIC toggles all static bodies, not just direct children of the world.

Python bindings

3. Added a free() method to MjrContext, rather than relying on the object destructor.
4. Enums now support arithmetic and bitwise operations with numbers.

Bug fixes

5. Fixed a rendering bug for planes, introduced in 2.1.2. This broke maze environments in dm_control.