Binding scope

Implement a binding scope object which is returned from bind()-functions. As long as this object is in scope, the GPU object remembers that it is bound and does not re-bind itself internally. Otherwise, a GPU object must bind itself when necessary.

Start by tracing the bind()-functions and the GPU object's internal binding paths to understand how binding state is currently tracked. Done means bind() returns a scope object whose lifetime prevents redundant internal rebinding, while GPU objects still bind themselves when that scope is absent.

Written by the indexing model from the issue text.