MDL : Support file syntax which precludes usage of ".." for relative paths

[bernardkwok]

This currently shows up for the "Example" materials which use "../../../Images/" syntax for image references.
Absolute file paths cannot be used (tried) and relative pathing does not appear to work even with the correct root search path.
There are examples of the current errors:

D:\Work\materialx\MaterialX-adsk-public-master\build_public_rel\source\MaterialXTest\resources\Materials\Examples\StandardSurface\standard_surface_brass_tiled\SR_brass1.genmdl.mdl(229,61): Error C292: strict relative file path contains too many directory names '..' D:\Work\materialx\MaterialX-adsk-public-master\build_public_rel\source\MaterialXTest\resources\Materials\Examples\StandardSurface\standard_surface_brass_tiled\SR_brass1.genmdl.mdl(229,61): Error C298: Unable to resolve file path '../../../Images/brass_color.jpg' D:\Work\materialx\MaterialX-adsk-public-master\build_public_rel\source\MaterialXTest\resources\Materials\Examples\StandardSurface\standard_surface_brass_tiled\SR_brass1.genmdl.mdl(230,64): Error C292: strict relative file path contains too many directory names '..' D:\Work\materialx\MaterialX-adsk-public-master\build_public_rel\source\MaterialXTest\resources\Materials\Examples\StandardSurface\standard_surface_brass_tiled\SR_brass1.genmdl.mdl(230,64): Error C298: Unable to resolve file path '../../../Images/brass_roughness.jpg'

It seems like the only way to address this is to find the path referenced via "../../../" and add that to the search path.
TBD if there is a better way to handle this.

Note that other files which use relative path such as "resource/Images/" do currently work so it's purely a binding issue and not related to shader generation.