The D2Scale property of MarkConfig was never initialized, leading to a
default value of 0 being passed to the D2 renderer. This eventually led
to calling chromedp.ScreenshotScale with a scale value of 0. Chrome
doesn't like this, and never finishes the sceenshot operation.
Fixes#610
Move a number of funcs/files in the top-level `main` package into a new
`util` package, so test logic can directly invoke functions like
RunMark(), etc. The main.go has been trimmed down to minimal sizing,
with former supporting funcs moved into `util` package, so they
can be run by unit tests.
Signed-off-by: Rich Scott <richscott@sent.com>
