inline some accessors and remove unused ouputPort stuff

All nodes have an output port named "image" of type NodePortType::FBOImage, no need to keep the custom output port stuff around unless the pipeline became multi-type and not just imaging.
go back to a bare destructor until specifics are understood
inline some accessors
add a method to see if a value is available for an input port key
expose amount port and default to 1.0
allow attachments to be defined at setup time

The number of attachments should be equal to the maximum number of FBOImage inputs on a node in the tree, plus one. This allows for ping-ponging.
theoretically support > 2 FBOImage input ports ⚠️

This is TOTALLY UNTESTED but I think the dependency calculation and evaluation stuff can handle more than 2 FBOImage input ports.
update in-progress content

Something here is better than nothing.