Fix problem showing exception in ~ToolLoopImpl()

We cannot have DrawingState::m_toolLoopManager = nullptr when we call
Console::showException() (as the Editor will continue sending some mouse
messages to the DrawingState).

src/app/tools/tool_loop.h

@@ -59,6 +59,7 @@ namespace app {
       enum Button { Left = 0, Right = 1 };
 
       virtual ~ToolLoop() { }
+      virtual void dispose() = 0;
 
       // Returns the tool to use to draw or use
       virtual Tool* getTool() = 0;

src/app/ui/editor/drawing_state.cpp

@@ -201,6 +201,9 @@ void DrawingState::onExposeSpritePixels(const gfx::Region& rgn)
 
 void DrawingState::destroyLoop()
 {
+  if (m_toolLoop)
+    m_toolLoop->dispose();
+
   delete m_toolLoopManager;
   delete m_toolLoop;
   m_toolLoopManager = NULL;

src/app/ui/editor/tool_loop_impl.cpp

@@ -184,7 +184,8 @@ public:
     m_offset.y = -y1;
   }
 
-  ~ToolLoopImpl()
+  // IToolLoop interface
+  void dispose() override
   {
     bool redraw = false;
 
@@ -230,7 +231,6 @@ public:
       update_screen_for_document(m_document);
   }
 
-  // IToolLoop interface
   tools::Tool* getTool() override { return m_tool; }
   Brush* getBrush() override { return m_brush; }
   Document* getDocument() override { return m_document; }