RenderInterface does not register static RenderTarget inputs

[j-o]

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_casts from AbstractSlot to Input<T> (lines 31-34) always fail.