Performance benchmark

[threefold3]

Add benches to verify performance of creating plots:

• a plot with many draw_pixel calls (e.g. mandelbrot plot)
• a plot with many text labels
  • see the effect of current need to create a font at each draw_text call
• a plot with many shapes (using draw_path, fill_polygon, draw_rect or draw_circle)
  • see if the need to create 2 vecs due to the interface of wxdragon::dc::DeviceContext::draw_polygon is a performance problem or not
• a plot with many lines
  • see if the need to create 2 vecs due to the interface of wxdragon::dc::DeviceContext::draw_lines is a performance problem or not