Revisit organisation of plot code

[pp-mo]

Following #3110 and #3111,
The current split between iris.plot._draw_2d_from_bounds and iris.plot._draw_2d_from_points seems awkward + should maybe consider re-integrating these 2, if the 2d coords extension touches both ?

[pp-mo]

Too hard / not necessarily better / no time
Gone to #3135