Support for getting frame timing statistics

I’d like to see the ability to get statistics about the frame timing, similar to the data IVRCompositor::GetFrameTiming() in OpenVR returns.
https://github.com/ValveSoftware/openvr/blob/master/headers/openvr.h#L1906-L1947

Having data about whether frames are getting reprojected seems to be required for properly implementing dynamic resolution scaling.