Addresses fixes #1871 by making `axis.set_scale` private and adding
a wrapper function with depreciation warning.
Spelling and import tweaks it axis.py.
wx backend should not attempt to render jpeg/tiff if PIL is present.

If PIL is present, the wx backend needs to allow the base class to get the
job done for print_jpeg and print_tiff, instead of trying to take over
duties itself.
Merge pull request #1917 from tacaswell/priv_axis.set_scale

Make `axis.set_scale` private
Merge remote-tracking branch 'upstream/v1.2.x'
Merge pull request #1770 from dhyams/wxbackend_tiff_jpeg_pil_fix

strange output from wx and wxagg when trying to render to JPEG or TIFF
Fix problem writing out meshes
implement path_effects for Line2D objects
modify patheffect_demo.py to demonstrate patheffects for Line2D objects
initial implementation of patheffect for collections
patheffect.draw_path_collection work with mixed renderer.
add tests for patheffects
Add a sketch path filter.
Fix oops in _path.cpp
Add xkcd() function to set up a number of parameters to create plots that look like xkcd.  Add an example, and a what's new.  Expose path effects as an rcParam so that they can be set globally.