dxt5-based parallax support

from @rextimmy
script (hlsl file):  defines a variant on parallaxOffset for use with dxt5 files, parallaxOffsetDxtnm
engine: when generating shaders, checks the material feature MFT_IsDXTnm to see if it is present in a given material definition. if so, it inserts a codeline into the shader referencing the above mentioned method instead of the former version.
immediate purpose: pretty much what it says on the tin.
long term purpose: @eightyeight brought up http://graphics.cs.brown.edu/games/SteepParallax/ as something we may want to look into. this tags the relevant area as well.