Add bump delta scale parameter in the Standalone shader

[vkoylazov]

http://forums.chaosgroup.com/showthread.php?87304-skin-shader-a-la-Alsurface-pleaseeee&p=705762#post705762

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One suggestion from the Maya side is that it would be good to include a bump delta scale parameter in the final version similar to the VrayMtl. The ability to lower the delta is really important when doing skin to get micro detail, otherwise the bump is too blurry and one needs to use displacement instead. In the meantime, we can of course pipe it through a VrayBumpMtl of course.
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