disable brush pressure averaging, implement single finger for camera rotation when touch draw is disabled

src/app.h

@@ -69,6 +69,7 @@ public:
     std::string doc_path;
     std::string doc_dir;
     std::string doc_filename;
+    bool has_stylus = false;
     float width;
     float height;
     bool keys[256];
@@ -132,6 +133,7 @@ public:
     bool key_up(kKey key);
     bool key_char(char key);
     void toggle_ui();
+    void set_stylus();
     void message_box(const std::string& title, const std::string& text);
 
     void rec_clear();