Add texture support for DistantLights

[jlanz]

Overview

Adds texture-driven importance sampling and evaluation for DistantLight, enabling textured distant lights with mip-level selection and correct pdf conversion from texture space to spherical-cap solid angle.

Changes:

• Adds UV mapping (equisolid-angle fisheye) between local direction and texture space for DistantLight and uses it for intersect(), sample(), and eval().
• Introduces mip-level computation for distant-light textures based on ray direction footprint and texel angular size.
• Extends the DistantLight ISPC/C++ shared member layout to support jacobian/pdf conversion and UV mapping constants.

Filed as internal issues MOONRAY-2595, MOONRAY-6048 (DreamWorks)