Add script to test rendering of exported CSG files.
Fix CSG output of offset().
Disable inf/nan cases for *.csg import tests.

The inf/nan tests fail when exporting CSG and rendering that output again
as currently inf/nan is written directly to the CSG file (e.g. r = inf)
which is not valid or even misleading in case a variable inf exists.
Merge branch 'master' into csg-import-tests
Put tests in correct test config
Merge branch 'master' into csg-import-tests
Merge branch 'master' into csg-import-tests
Merge pull request #797 from openscad/csg-import-tests

Csg import tests