Fix #1725 Texture can now be plugged directly on socket

[julienduroure]

Fix #1725 Texture can now be plugged directly on socket

[julienduroure]

If No SeparateColor node found, so take the specification channel that is needed
So export is correct if user plug the texture directly to the socket
Currently on master, the R channel is taken by default.

This still need to be fully tested to be sure there is no regression

[julienduroure]

TODO :

• Crash when texture is plugged on Color, but socket is int (and should be linked to Alpha)