fix stroke preview, use hmd pose instead of eye, fix vr fullscreen mode

src/app.h

@@ -80,6 +80,7 @@ public:
     bool keys[256];
     bool redraw = true;
     bool animate = false;
+    bool ui_visible = true;
     glm::mat4 vr_rot{ 0 };
     glm::mat4 vr_uirot{ 0 };
     glm::vec2 cursor{ 0, 0 };
@@ -124,7 +125,7 @@ public:
     void clear();
     void tick(float dt);
     void update(float dt);
-    void vr_draw(const glm::mat4& proj, const glm::mat4& view);
+    void vr_draw(const glm::mat4& proj, const glm::mat4& view, const glm::mat4& pose);
     void async_start();
     void async_update();
     void async_redraw();