PlanarMapping2D: fix bug in computation of partial derivatives

We need to transform dpdx and dpdy to texture space first... (Not doing that
could lead to overblurring or aliasing, depending on the circumstances.)

src/pbrt/textures.h

@@ -187,9 +187,13 @@ class PlanarMapping2D {
     PBRT_CPU_GPU
     Point2f Map(TextureEvalContext ctx, Vector2f *dstdx, Vector2f *dstdy) const {
         Vector3f vec(textureFromRender(ctx.p));
-        *dstdx = Vector2f(Dot(ctx.dpdx, vs), Dot(ctx.dpdx, vt));
-        *dstdy = Vector2f(Dot(ctx.dpdy, vs), Dot(ctx.dpdy, vt));
-        return {ds + Dot(vec, vs), dt + Dot(vec, vt)};
+        // Initialize partial derivatives of planar mapping $(s,t)$ coordinates
+        Vector3f dpdx = textureFromRender(ctx.dpdx);
+        Vector3f dpdy = textureFromRender(ctx.dpdy);
+        *dstdx = Vector2f(Dot(dpdx, vs), Dot(dpdx, vt));
+        *dstdy = Vector2f(Dot(dpdy, vs), Dot(dpdy, vt));
+
+        return Point2f(ds + Dot(vec, vs), dt + Dot(vec, vt));
     }
 
     std::string ToString() const;