Combining texture shaders referring to the same texture

[jonathan-meier]

Currently, in both PRTMaterialNode and ArnoldMaterialNode we use a separate texture shaders for each map, regardless of whether the textures are actually the same or different.

Task:

• Create only one texture shader for different maps using the same texture file.

(Do we need to support the case where a single texture file is used in multiple maps with different UV transforms?)

[mistafunk]

we should also share texture nodes which are connected to different shader inputs (e.g. color and transparency etc)