RenderInterface does not register static RenderTarget inputs

Currently, the RenderInterface does not register static RenderTarget inputs, as `Stage::addInput` (and thus the signal handler in `RenderInterface`) is called from the constructor of `Slot` for static inputs, which means that the `Input` is not fully constructed yet. Consequently, the `dynamic_cast`s from `AbstractSlot` to `Input` (lines 31-34) always fail.

Locate RenderInterface, Stage::addInput, and the Slot constructor, then inspect the signal handler and the dynamic_casts at lines 31-34. Trace static RenderTarget construction to confirm why the Input is incomplete when registration occurs. Done means static RenderTarget inputs are registered successfully without the failed casts.

Written by the indexing model from the issue text.