PixelPaint: Inflate update rect for PenTool

Previously using PenTool while zoomed in could cause rendering glitches
since the update rect was misaligned with the image, most likely because
of rounding errors.
Loosening the rect with 1 pixel on either side takes care of this.

Userland/Applications/PixelPaint/PenTool.cpp

@@ -31,7 +31,7 @@ void PenTool::on_mousedown(Layer& layer, GUI::MouseEvent& event, GUI::MouseEvent
 
     GUI::Painter painter(layer.bitmap());
     painter.draw_line(event.position(), event.position(), m_editor->color_for(event), m_thickness);
-    layer.did_modify_bitmap(Gfx::IntRect::centered_on(event.position(), Gfx::IntSize { m_thickness, m_thickness }));
+    layer.did_modify_bitmap(Gfx::IntRect::centered_on(event.position(), Gfx::IntSize { m_thickness + 2, m_thickness + 2 }));
     m_last_drawing_event_position = event.position();
 }
 
@@ -57,7 +57,7 @@ void PenTool::on_mousemove(Layer& layer, GUI::MouseEvent& event, GUI::MouseEvent
         painter.draw_line(event.position(), event.position(), m_editor->color_for(event), m_thickness);
         changed_rect = Gfx::IntRect::from_two_points(event.position(), event.position());
     }
-    changed_rect.inflate(m_thickness, m_thickness);
+    changed_rect.inflate(m_thickness + 2, m_thickness + 2);
     layer.did_modify_bitmap(changed_rect);
 
     m_last_drawing_event_position = event.position();