BIsHmdPresent always returns true

[zeuxcg]

Calling VR_IsHmdPresent (before calling VR_Init) now seems to always return true if SteamVR is installed at all - even if no headset is plugged in or no headset has ever been plugged in on the system in question.

This used to work around a month ago; I'm not sure which specific SteamVR update changed this behavior - it looks like it's been happening since Feb 14th at least but I can't date the issue any earlier than that.

This happens on latest non-beta SteamVR (v1485823399) with OpenVR 1.0.5 and OpenVR 1.0.6. The expected behavior is that the return value is false unless a headset is actually plugged in, which is how the function behaved in January.

The reason this is an issue for us is that VR_Init starts SteamVR unconditionally now; we need to check if the user has a VR device installed and plugged in *without* starting up SteamVR - it used to be possible with VR_IsHmdPresent but since it now returns true we have to try to call VR_Init (which fails with HmdNotPresent, as expected).