Blitting vectorscope plotting

[KinMaynard]

Currently the library is bottlenecked by calls to

fig.canvas.draw_idle()

These possibly occur inside vectorscope on zoom/pan. These are apparent when zooming, panning or using scale/side & reset buttons. (The reset buttons are relimiting axis but may also be redrawing them?).

According to matplotlib docs using blitting will help performance issues

Blitting steps:

• First we must use backend that supports blitting (macosx backend does not)
• Prepare constant background
• Mark changing artists as animated
• Draw figure excluding animated artists
• save copy of RBGA buffer
• Render images
• restore copy of RGBA buffer
• redraw animated artists using Axes.draw_artist/Figure.draw_artist
• show resulting image to screen

Relevant matplotlib docs

Blog post about speeding up matplotlib