granularity is all GBuffers, could be per texture but might not be worth the tracking
* disabled RenderTargetPoolEvents tracking for editor rendering to get a more readable output
[CL 2076060 by Martin Mittring in Main branch]
#ttp 331829 - Mobile Preview in Editor
#proj Engine
#branch UE4
#summary First pass of changes to remove GRHIFeatureLevel, allowing for multiple scenes with different feature levels to exist. First step towards dynamic mobile preview in the editor.
#change Added GMaxRHIFeature level which contains the highest supported feature level on this machine
#change Added a cvar "r.FeatureLevelPreview" which, when set to 1, enables a feature level selection in the quick settings menu. Not wired up to anything at the minute
#change Added a feature level variable to FScene, which is currently initialized to whatever GRHIFeatureLevel is. Accessed with GetFeatureLevel()
#change Added a helper accessor GetFeatureLevel() to FSceneView which gets the feature level from the relevant scene. Purely to reduce the amount of typing needed to get to a feature level
#change Many changes across the renderer to try and read the current feature level from the view or scene. Not everything is done yet, but this is almost all of the "easy" stuff. As there is still just a global feature level in the engine, nothing should change.
[CL 2066905 by Graeme Thornton in Main branch]
