BUG: Recompute Yvv GPU coefficients against actual input spacing

SetSigmaArray() reads spacing from GetOutput()->GetSpacing() at
user-config time, where the output DataObject still carries the
default unit spacing (1.0 in every dimension). The CPU sibling
RecursiveLineYvvGaussianImageFilter recomputes the b-coefficients
and M-matrix in BeforeThreadedGenerateData() using the actual
input spacing.

Add the equivalent recomputation at the entry of GPUGenerateData()
so non-unit-spacing images (e.g. CT with 0.5 mm voxels) produce
correct Gaussian kernels on the GPU path.

Author: hjmjohnson

Modules/Filtering/SmoothingRecursiveYvvGaussianFilter/include/itkGPUSmoothingRecursiveYvvGaussianImageFilter.hxx

@@ -292,6 +292,16 @@ GPUSmoothingRecursiveYvvGaussianImageFilter<TInputImage, TOutputImage>::GPUGener
     this->BuildKernel();
   }
 
+  // Recompute the b-coefficients and M-matrix against the actual input spacing.
+  // SetSigmaArray() reads GetOutput()->GetSpacing() before Update(), where the
+  // output DataObject still carries the default unit spacing; the CPU sibling
+  // recomputes per-call in BeforeThreadedGenerateData (see #6243).
+  const typename InputImageType::SpacingType & inputSpacing = this->GetInput()->GetSpacing();
+  if (inputSpacing[0] != 0)
+  {
+    this->SetUp(inputSpacing[0]);
+  }
+
   constexpr unsigned int X = 0;
   constexpr unsigned int Y = 1;
   constexpr unsigned int Z = 2;