Re-use texture input nodes

[tdapper]

Quoting myself from https://github.com/Thicket-Blender/thicket/issues/32:

One possible optimization (not sure if it has any performance implications, but it may simplify/clarify the material graph): Perhaps keep a list of texture input nodes around and just re-use a node when a texture is used multiple times (with the same color space setting). In the example above, e.g. the input node for the front BSDF color could be re-used as input to the Mix BSDF node.