GRHISupportsRHIThread

Whether or not the RHI supports an RHI thread. Requirements for RHI thread Microresources (those in RHIStaticStates.h) need to be able to be created by any thread at any time and be able to work with a radically simplified rhi resource lifecycle. CreateSamplerState, CreateRasterizerState, CreateDepthStencilState, CreateBlendState CreateUniformBuffer needs to be threadsafe GetRenderQueryResult should be threadsafe, but this isn't required. If it isn't threadsafe, then you need to flush yourself in the RHI GetViewportBackBuffer and AdvanceFrameForGetViewportBackBuffer need to be threadsafe and need to support the fact that the render thread has a different concept of "current backbuffer" than the RHI thread. Without an RHIThread this is moot due to the next two items. AdvanceFrameForGetViewportBackBuffer needs be added as an RHI method and this needs to work with GetViewportBackBuffer to give the render thread the right back buffer even though many commands relating to the beginning and end of the frame are queued. BeginDrawingViewport, and 5 or so other frame advance methods are queued with an RHIThread. Without an RHIThread, these just flush internally.