Nullgl

allow build without use of OpenGL through a special test build with 'cmake .. -DNULLGL=1'

resulting openscad_nogui binary will output 3d formats like STL but cant do OpenCSG/png/etc. 

the changes to PolySet are not really changes, just move all openGL functions to the bottom of the file and wrap with a ifdef

there are some changes to CMakeTests and elsewhere to deal with a few bugaboos related to trying to build without OpenCSG (which of course, depending on GL, is disallowed for a NULLGL build)

ImageMagick is not used