A few implementations of Render() contain const-casts nested within its
their call hierarchy to get around the fact that this function is marked
const. We can just make the member function non-const to allow removal
of these casts in follow up changes.
This member function alters instance state in a few implementations, so
it shouldn't be made const.
The state manager parameter also shouldn't be const. Retrieved data
from the post constructed instance is further modified in some
implementations. This removes the constness on this parameter in order
to fix more const_cast usages in a follow-up change.
