Fix SDL GPU D3D12 Descriptor Heap leak

When descriptor leaks overflow, the D3D12 SDL GPU renderer fetches more from the pool, but never returns them, which eventually causes an "out of memory" crash.

src/gpu/d3d12/SDL_gpu_d3d12.c

@@ -4885,7 +4885,14 @@ static void D3D12_INTERNAL_SetGPUDescriptorHeaps(D3D12CommandBuffer *commandBuff
     viewHeap = D3D12_INTERNAL_AcquireGPUDescriptorHeapFromPool(commandBuffer, D3D12_DESCRIPTOR_HEAP_TYPE_CBV_SRV_UAV);
     samplerHeap = D3D12_INTERNAL_AcquireGPUDescriptorHeapFromPool(commandBuffer, D3D12_DESCRIPTOR_HEAP_TYPE_SAMPLER);
 
+    if (commandBuffer->gpuDescriptorHeaps[0] != NULL) {
+        D3D12_INTERNAL_ReturnGPUDescriptorHeapToPool(commandBuffer->renderer, commandBuffer->gpuDescriptorHeaps[0]);
+    }
     commandBuffer->gpuDescriptorHeaps[0] = viewHeap;
+
+    if (commandBuffer->gpuDescriptorHeaps[1] != NULL) {
+        D3D12_INTERNAL_ReturnGPUDescriptorHeapToPool(commandBuffer->renderer, commandBuffer->gpuDescriptorHeaps[1]);
+    }
     commandBuffer->gpuDescriptorHeaps[1] = samplerHeap;
 
     heaps[0] = viewHeap->handle;