Directx12 crashes with OOM

In editor, when running a task that does not issue a BeginFrame/EndFrame for a long time, the GPU will eventually run out of memory. 

See UDN: https://udn.unrealengine.com/questions/431450/directx12-oom-issue.html

1. Enable DirectX 12 by adding the "-DX12" argument to a shortcut target of the Editor.
   1. Right click the shortcut
   2. Click Properties
   3. In the Target field add "-DX12" to the end
2. Open a project with a large amount of assets. (I simply created a project and added all feature packs and started content)
3. Right-click a folder with a lot of assets pick the option 'Resave All' (I just used the content folder as a whole)
4. A slow task message box will appear

Result: After 10+minutes, there will be a large spike in VRAM usage. It is possible that this leads to a crash.

During the task, Slate will generate lots of 'FD3D12DynamicHeapAllocator::AllocUploadResource' for updating Vertex/Index buffers, but those are only released on EndFrame, leading to an OOM.