GPU/wavefront: compute dpdx/dpdy for VisibleSurface at first hit

In turn, this gives us valid dzdx and dzdy values with GBuffer film.

src/pbrt/wavefront/integrator.cpp

@@ -83,6 +83,7 @@ WavefrontPathIntegrator::WavefrontPathIntegrator(Allocator alloc, ParsedScene &s
 
     sampler = Sampler::Create(scene.sampler.name, scene.sampler.parameters,
                               film.FullResolution(), &scene.sampler.loc, alloc);
+    samplesPerPixel = sampler.SamplesPerPixel();
 
     Medium cameraMedium = findMedium(scene.camera.medium, &scene.camera.loc);
     camera = Camera::Create(scene.camera.name, scene.camera.parameters, cameraMedium,
@@ -347,7 +348,6 @@ Float WavefrontPathIntegrator::Render() {
     Timer timer;
     Vector2i resolution = film.PixelBounds().Diagonal();
     Bounds2i pixelBounds = film.PixelBounds();
-    int spp = sampler.SamplesPerPixel();
     // Launch thread to copy image for display server, if enabled
     RGB *displayRGB = nullptr, *displayRGBHost = nullptr;
     std::atomic<bool> exitCopyThread{false};
@@ -429,7 +429,7 @@ Float WavefrontPathIntegrator::Render() {
                 });
     }
 
-    int firstSampleIndex = 0, lastSampleIndex = spp;
+    int firstSampleIndex = 0, lastSampleIndex = samplesPerPixel;
     // Update sample index range based on debug start, if provided
     if (!Options->debugStart.empty()) {
         std::vector<int> values = SplitStringToInts(Options->debugStart, ',');

src/pbrt/wavefront/integrator.h

@@ -148,7 +148,7 @@ class WavefrontPathIntegrator {
     pstd::vector<Light> *envLights;
     LightSampler lightSampler;
 
-    int maxDepth;
+    int maxDepth, samplesPerPixel;
     bool regularize;
 
     int scanlinesPerPass, maxQueueSize;

src/pbrt/wavefront/surfscatter.cpp

@@ -117,6 +117,8 @@ void WavefrontPathIntegrator::EvaluateMaterialAndBSDF(TextureEvaluator texEval,
                 intr.wo = w.wo;
                 intr.time = w.time;
 
+                camera.ApproximatedPdxy(intr, samplesPerPixel);
+
                 // Estimate BSDF's albedo
                 constexpr int nRhoSamples = 16;
                 SampledSpectrum rho(0.f);