matplotlib priorities

Regarding "Performance - Faster point and symbol plots"

The RenderBase class has the draw_point() method, I think it should have
a "draw_points()" method added to it. Then points would be similar to
lines which have draw_line() and draw_lines() methods.

It should then be easy to gain a speedup (in GTK+ at least) by calling
the relevant GDK C library function, instead of using a Python loop
calling draw_point().

Existing methods:
draw_line() calls drawable.draw_line()
draw_lines() calls drawable.draw_lines()
draw_point() calls drawable.draw_point()

New method:
draw_points() calls drawable.draw_points()

Line2D._draw_pixel() should become much faster.