Removes build-time transformation from FIG to PNG or EPS and instead checks in the PNG files for our handful of figures. This then removes the need for Fig2Dev, Ghostscript, and ImageMagick.
Fixes Doxygen installation on Appveyor by directly downloading it rather than trying to use Chocolatey.
Fixes identification of whether Doxygen is Cygwin or not by changing the test to try to write to a UNIX path as the old test failed on newer Doxygen versions.
Fixes #3235 (closed) Fixes #4000 (closed)