Fixed Foveated Rendering for SteamVR games - alpha v0.2

Changelog:

• added Variable Rate Shading (VRS) as an alternative implementation for achieving fixed foveated rendering. This variant is only available on NVIDIA RTX and GTX 16xx cards, unlike the default implementation via radial density masking (RDM). In general, VRS will look better than RDM in the reduced resolution rings, and there is a chance that VRS will work with more games than RDM.
• fixed a small bug in the RDM reconstruction that left visible artifacts at the transition between rings.
• added more hotkeys and made their keys configurable in the config file.
• added hotkey to take a screenshot of the final render.

Fixed Foveated Rendering for SteamVR games - alpha v0.1

First public alpha test release :)