Add low-level CGL context management for macOS.

This allows rendering from non-main threads as the context is not tied to a Cocoa window.

Fixes #742
Fixes #798

PiperOrigin-RevId: 523144359
Change-Id: I98432debdd034814c313eda94e38c248c3c1f76c

python/mujoco/gl_context.py

@@ -40,6 +40,9 @@ if _MUJOCO_GL not in ('disable', 'disabled', 'off', 'false', '0'):
   elif _SYSTEM == 'Linux' and _MUJOCO_GL == 'egl':
     from mujoco.egl import GLContext as _GLContext
     GLContext = _GLContext
+  elif _SYSTEM == 'Darwin':
+    from mujoco.cgl import GLContext as _GLContext
+    GLContext = _GLContext
   else:
     from mujoco.glfw import GLContext as _GLContext
     GLContext = _GLContext