review vr stuff, fix controller-ui ray casting

src/hmd.h

@@ -40,6 +40,8 @@ struct Vive
     glm::mat4 m_view[2];
     glm::mat4 m_proj[2];
     glm::mat4 m_pose;
+    glm::vec3 m_initial_position;
+    bool m_position_valid = false;
     bool m_active = false;
     std::function<void(const glm::mat4& m_proj, const glm::mat4& m_view, const glm::mat4& m_pose)> on_draw = nullptr;
     std::function<void(const ViveController&, ViveController::kButton, ViveController::kAction, glm::vec2 axis)> on_button = nullptr;