implement eraser detection on windows

src/canvas_modes.h

@@ -37,6 +37,7 @@ public:
 
 class CanvasModePen : public CanvasMode
 {
+    bool m_touching = true;
     bool m_dragging = false;
     glm::vec2 m_pan_start;
     glm::vec2 m_cur_pos;
@@ -50,6 +51,7 @@ class CanvasModePen : public CanvasMode
     // resizing the tip
     bool m_resizing = false;
 public:
+    CanvasModePen() = default;
     virtual void on_MouseEvent(MouseEvent* me, glm::vec2& loc) override;
     virtual void on_Draw(const glm::mat4& ortho, const glm::mat4& proj, const glm::mat4& camera) override;
     virtual void enter() override;