add spacebar to rotate the view

src/canvas_modes.h

@@ -56,9 +56,11 @@ public:
 class CanvasModeBasicCamera : public CanvasMode
 {
     bool m_draggingR = false;
+    bool m_draggingL = false;
     bool m_draggingT = false; // touch drag
     bool m_zooming = false;
     glm::vec2 m_dragR_start;
+    glm::vec2 m_dragL_start;
     glm::vec2 m_pan_start;
     float m_fov_start;
     float m_camera_fov;