Try two at disk parameterization and partial derivatives.

The fix from issue mmp#154 led to a different bug (issue mmp#183). This switches
back to the old partial derivative vectors via adjusting the disk
parameterization. (Now (0,0) is on the outer ring, which isn't great, but
seems like the least worst option.)

src/shapes/disk.cpp

@@ -73,8 +73,7 @@ bool Disk::Intersect(const Ray &r, Float *tHit, SurfaceInteraction *isect,
     // Find parametric representation of disk hit
     Float u = phi / phiMax;
     Float rHit = std::sqrt(dist2);
-    Float oneMinusV = ((rHit - innerRadius) / (radius - innerRadius));
-    Float v = 1 - oneMinusV;
+    Float v = (radius - rHit) / (radius - innerRadius);
     Vector3f dpdu(-phiMax * pHit.y, phiMax * pHit.x, 0);
     Vector3f dpdv =
         Vector3f(pHit.x, pHit.y, 0.) * (innerRadius - radius) / rHit;