preprocessing: a buffer someone else page-locked is already what we wanted

The uncompressed-image upload page-locks the caller's bytes, which is right when
the caller allocated them for us and wrong when it did not: the online receiver
registers its image buffer once and hands out ranges inside it, and registering
a range a second time is an error rather than a no-op, so the first frame
aborted the collection.

An already-registered range needs nothing done to it - the upload is a DMA
either way - and it is not ours to unregister when this object is destroyed, so
it is deliberately not remembered.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

image_analysis/image_preprocessing/ImagePreprocessorGPU.cu

@@ -352,7 +352,16 @@ void ImagePreprocessorGPU::PinInputRegion(const void *ptr, size_t bytes) {
         cuda_err(cudaHostUnregister(const_cast<void *>(pinned_input)));
     pinned_input = nullptr;
     pinned_input_bytes = 0;
-    cuda_err(cudaHostRegister(const_cast<void *>(ptr), bytes, cudaHostRegisterDefault));
+    // The buffer may already be page-locked by whoever owns it - the receiver registers its image
+    // buffer once and hands out ranges inside it - and registering a range twice is an error rather
+    // than a no-op. There is nothing to do in that case: the upload is already a DMA, and the
+    // region is not ours to unregister later, so it is deliberately not remembered.
+    const auto err = cudaHostRegister(const_cast<void *>(ptr), bytes, cudaHostRegisterDefault);
+    if (err == cudaErrorHostMemoryAlreadyRegistered) {
+        cudaGetLastError();
+        return;
+    }
+    cuda_err(err);
     pinned_input = ptr;
     pinned_input_bytes = bytes;
 }