Free the GL context when OffScreenRenderer.release() is called.
Free the GL context when OffScreenRenderer.release() is called.

After the previous change, the surface would only be released if there was an active MjrContext. This seems to be the case in practice, but it since it's possible for OffScreenRenderer to exist without an active MjrContext (before rendering), it makes sense to manage the GL context separately.

PiperOrigin-RevId: 435647965
Change-Id: I58da564dc4a7e04d77a2652c7511f03351fb84c6

deepmind/dm_control