ShaderGraph improvements

[trethaller]

Let's collect here the various issues and missing features that currently prevent or hider usage of shader graphs in production:

• Nesting subgraphes: When putting graph A inside graph B inside graph C, the texture default values of A are no longer present in C, and we get an error that says "default value of ... parameter not found"
• Need access to depthMap as global input
• Would be nice to be able to input values directly for subgraphes parameters, currently the only way is to plug in constant values, which is not convenient
• Links splines sometimes draw inverted