Add VapourSynth Environment and Node Access

[BoatsMcGee]

Adds a new function to return both the Environment and Video Node. This avoids both mutably and immutably borrowing which makes the below example possible. node_modifier is now public to allow updating it after instantiation.

Also allows specifying an output_index in VapoursynthDecoder (defaults to 0, maintaining previous behavior) to select a specific Video Node when a script/environment produces multiple nodes.

Example

let (env, reference_node) = decoder.get_vapoursynth();
let core = get_core(env);
let distorted_node = import_video(core);
let compared_node = compare_nodes(reference_node, distorted_node);