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 #1770 from dhyams/wxbackend_tiff_jpeg_pil_fix

strange output from wx and wxagg when trying to render to JPEG or TIFF