Support DiscreteNorm in scatter plot colorbars

[lukelbd]

Resolves#144. Now ax.scatter() accepts the exact same keyword arguments used to generate a DiscreteNorm as methods like ax.contourf() and ax.pcolormesh() -- if it detects that you passed an array of data values for the c argument.

[lukelbd]

Decided to also throw in a bar plot bugfix in this PR -- it turns out #156 did not entirely solve the issues with grouped bar positions

[lukelbd]

...and added a bugfix for the qt backend, because the examples I tested this on kept triggering recursive paint errors. Turned out that when setting the figure size after adding new panels to the figure (e.g. after making colorbars with ax.colorbar(m, loc='left')), this figure resize was triggering the canvas pre-processor, which triggered the tight layout algorithm and another figure resize, etc., etc. Added a breakout condition in the preprocessor that prevents this.