WIP: Feature/#294 untangle hmd open vr

This is not necessarily finished, and may be just a proposal going forward (depending on the scope of the issue). Tell me what you think.

The phx::HMD was essentially split into its OpenVR functionality (phx::OpenVRUnit) and the head representation (phx::Head), which can also be used in a CAVE or a Non-OpenVR HMD implementation (the head representation doesn't actually do a lot, and I don't know how much sense this makes, as, e.g., its transformation would probably be accessed via a head entity). Similar things could/should be done with controllers, etc., although this also has to be revamped when we "solve" input.

phx::OpenVRUnit could also be generalized into an HMDUnit that is derived from, but I don't see us implementing Non-OpenVR-based HMD support any time soon.

@jwendt @mbellgardt

demos/viewer/src/navigation_behavior.cpp

@@ -31,21 +31,20 @@
 #include "phx/display_system.hpp"
 #include "phx/engine.hpp"
 #include "phx/entity.hpp"
-#include "phx/hmd.hpp"
+#include "phx/openvr_unit.hpp"
 #include "phx/scene.hpp"
 #include "phx/transform.hpp"
 
 void NavigationBehavior::OnUpdate() {
-  // If there exists an HMD and a transform.
-  const auto hmd = GetEntity()
-                       ->GetScene()
-                       ->GetEngine()
-                       ->GetSystem<phx::DisplaySystem>()
-                       ->GetHMD();
+  const auto openvr_unit = GetEntity()
+                               ->GetScene()
+                               ->GetEngine()
+                               ->GetSystem<phx::DisplaySystem>()
+                               ->GetOpenVRUnit();
   const auto transform = GetEntity()->GetFirstComponent<phx::Transform>();
-  if (!hmd || !transform) return;
+  if (!openvr_unit || !transform) return;
 
-  auto indices = hmd->GetControllerIndices();
+  auto indices = openvr_unit->GetControllerIndices();
 
   for (auto i = 0u; i < indices.size(); ++i) {
     vr::VRControllerState_t controller_state;
@@ -56,10 +55,12 @@ void NavigationBehavior::OnUpdate() {
     if (controller_state.ulButtonTouched &
         vr::ButtonMaskFromId(vr::EVRButtonId::k_EButton_SteamVR_Trigger))
       transform->Translate(
-          -0.05F * glm::column(hmd->GetRightControllerTransformation(), 2));
+          -0.05F *
+          glm::column(openvr_unit->GetRightControllerTransformation(), 2));
     if (controller_state.ulButtonTouched &
         vr::ButtonMaskFromId(vr::EVRButtonId::k_EButton_SteamVR_Touchpad))
       transform->Translate(
-          0.05F * glm::column(hmd->GetRightControllerTransformation(), 2));
+          0.05F *
+          glm::column(openvr_unit->GetRightControllerTransformation(), 2));
   }
 }