BUG: Driver Direct mode should not be doing lens distortion

[TheDeveloperGuy]

In the latest SteamVR Beta, there is a bug where the runtime calls ComputeDistortion() when the headset is using Driver Direct mode (IVRDriverDirectModeComponent). This is new and incorrect behaviour. In this driver mode, the driver is responsible for lens distortion correction.

Most headset drivers don't support multiple driver modes, and it is possible to work around this bug in the headset driver. Which I will do, because no one (at Valve) will ever read this, or fix the issue.