add openvr support, switch to 8.1 sdk, prepare for vr ui

src/app.h

@@ -6,6 +6,7 @@
 #include "texture.h"
 #include "layout.h"
 #include "font.h"
+#include "rtt.h"
 #include "node_message_box.h"
 #include "node_settings.h"
 #include "node_popup_menu.h"
@@ -46,7 +47,11 @@ public:
     std::condition_variable rec_cv;
     std::deque<std::unique_ptr<uint8_t[]>> rec_frames;
     
+    RTT uirtt;
     Sampler sampler;
+    Sampler sampler_stencil;
+    Sampler sampler_linear;
+    ui::Plane m_face_plane;
     LayoutManager layout;
     NodeMessageBox* msgbox;
     NodeSettings* settings;
@@ -116,6 +121,7 @@ public:
     void clear();
     void tick(float dt);
     void update(float dt);
+    void vr_draw(const glm::mat4& proj, const glm::mat4& view);
     void async_start();
     void async_update();
     void async_redraw();