DiskLight / RectLight with spread < 1: intersect() reports a different pdf than sample() #330

Two related problems in lib/rendering/pbr/light/DiskLight.cc and RectLight.cc (same in the ISPC versions), version 2026.29.1.

1. DiskLight: bounding box instead of the disk

Reproduce. Glossy floor (roughness 0.3) under a disk light, max_depth 1:

Cause. DiskLight::sample() uses mInvArea (1 / (pi R^2)) when the whole disk lies inside the spread region (lightIsInsideSpread), and the overlap rectangle otherwise. DiskLight::intersect() always calls getOverlapBounds, which in the first case returns the 2R x 2R bounding box, so the hit pdf is 1 / (4 R^2). The MIS weights of light and BSDF samples then do not sum to one.

Fix (untested). Use the same case distinction in intersect() as in sample().

2. Reverse and two-sided lights: signed distance

Reproduce. The same floor under a light with sidedness reverse and spread 0.8:

Cause. intersect() computes float spreadLength = localP.z * mTanSpreadTheta; with the signed localP.z, which is negative on the reverse side. getOverlapBounds then finds no overlap and returns false without writing width and height; the return value is not checked and the pdf is computed from them. sample() uses the sign-corrected plane distance and is fine.

Fix (untested). Use abs(localP.z), and check the return value of getOverlapBounds.