Add workaround for broken aggdraw.Font usage in satpy/pycoast

[djhoese]

This addresses the pytroll/pycoast#43 issue. In summary, Satpy tried to delay calls to pycoast by using the apply_pil method on trollimage's XRImage class. In some use cases this could involve passing an aggdraw Font object to the dask Delayed function. As part of the tokenization of the arguments dask calls type(obj) on the Font object which causes a segmentation fault because aggdraw.Font objects are not normal python types.

The solution in this PR is a complete hack, but works. The long term solution is to fix aggdraws classes but that is a huge problem to solve. Another solution would be to update pycoast to pass the individual components of a font (path, color, size, opacity) but that is also a pretty huge task (easier than aggdraw) if it is done right. This solution, essentially an if statement, should be removed when a better version of aggdraw is available.

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)

CC @peters77

[coveralls]

Coverage increased (+0.09%) to 90.83% when pulling 120d5ce on djhoese:bugfix-aggdraw-font into 50d3262 on pytroll:master.