Add multioutput type check in getShaderNodes

[xipaja]

This fixes #2529

This issue stems from validation that the Shader node’s output type matches the Material node’s input type. It doesn't note a multioutput node as a matching type and returns an empty shaderNodes vector to the Graph Editor. It never creates a Material due to not receiving shaderNodes, leading to the empty Render View in the Editor. This then leads to a lack of renderable elements in the Viewer.

This change will check available activeOutputs for multiOutput nodes to ensure at least one active output is compatible with the input of the Material node (for example, surfaceshader to surfaceshader). It will maintain the previous behavior otherwise.

Tested with the test files from the original issue:

[jstone-lucasfilm]

This looks like a good improvement to me, @xipaja! I'll give others a chance to weigh in as well, but I'd have no objections to including this in MaterialXCore.