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

src/canvas_modes.h

@@ -25,6 +25,7 @@ public:
 class CanvasModeBasicCamera : public CanvasMode
 {
     bool m_draggingR = false;
+    bool m_draggingT = false; // touch drag
     glm::vec2 m_dragR_start;
     glm::vec2 m_pan_start;
     float m_camera_fov;