SItoA 7.3.7.1

Be sure to read the Arnold core release notes from version 7.3.6.0 to 7.3.7.1 to see all the performance improvements, bug fixes and other changes that comes "for free" with the updated core.

SItoA specific improvements

• Added OpenPBR to Arnold Shaders menu
• Made OpenPBR the default shader in Scene Material
• Added trasmission_shadow_density to standard_surface and openpbr_surface

Incompatible / Behavior changes

• None that I know of.

Bugfixes

• None

Build system fixes / changes:

• None

Passes testsuite.