D3D HUNG crash - FD3D11DynamicRHI::RHIReadSurfaceFloatData()

This is one of the widespread D3D Hung crashes that have existed since 4.14. This particular callstack has affected around 300 users. The issue has been successfully reproduced on a home PC.

Dxdiag and log with -d3ddebug are attached

This crash is hardware-dependent and does not reproduce on all systems. Successfully reproduced on a home PC; 4/4 attempts.

1. Make a new blank project with Maximum Settings or open an existing project
2. New Level > VR-Basic (theoretically any map with some reflective assets could work)
3. Add a Sphere Reflection Capture to the level, move it around swiftly (in 'Lit' mode) for a few seconds (~20 seconds).
4. If you don't crash, add up to 4 more Sphere Reflection Captures, swiftly moving each one around for a few seconds as you add.

The crash seems to require a built-up strain be placed on the GPU. On my home PC, having one Reflection Sphere in the level only crashed after I had swapped out materials on assets, PIE'd, and moved around lights. If I didn't take any other prior actions, then I needed between 2 and 4 Reflection Sphere before the crash would occur.

Fatal error: [Link Removed] [Line: 176] Unreal Engine is exiting due to D3D device being lost. (Error: 0x887A0006 - 'HUNG')

UE4Editor_D3D11RHI!TerminateOnDeviceRemoved() d3d11util.cpp:176
UE4Editor_D3D11RHI!VerifyD3D11Result() d3d11util.cpp:225
UE4Editor_D3D11RHI!FD3D11DynamicRHI::RHIReadSurfaceFloatData() d3d11rendertarget.cpp:1176
UE4Editor_RHI!FRHICommandListImmediate::ReadSurfaceFloatData() rhicommandlist.h:3025
UE4Editor_Renderer!ComputeSingleAverageBrightnessFromCubemap() reflectionenvironmentcapture.cpp:316
UE4Editor_Renderer!ComputeAverageBrightness() reflectionenvironmentcapture.cpp:379
UE4Editor_Renderer!TGraphTask<`FScene::UpdateReflectionCaptureContents'::`19'::EURCMacro_FilterCommand>::ExecuteTask() taskgraphinterfaces.h:868
UE4Editor_Core!FNamedTaskThread::ProcessTasksNamedThread() taskgraph.cpp:932
UE4Editor_Core!FNamedTaskThread::ProcessTasksUntilQuit() taskgraph.cpp:679
UE4Editor_RenderCore!RenderingThreadMain() renderingthread.cpp:320
UE4Editor_RenderCore!FRenderingThread::Run() renderingthread.cpp:454
UE4Editor_Core!FRunnableThreadWin::Run() windowsrunnablethread.cpp:74