add test imgs for offpngtest and stlpngtest. updates to test_cmd and test_pretty_print
to handle missing expected images better and properly report img comparer.
tweak diffpng
add more expected images. remove accidentally added intermediate files.
run the diffpng image comparer without creating an output image, for speed
update numerous tests to have proper expected images. fix resize-2 test
update resize-2d tests
turn off debuggin
fix missing ThrownTogether rendering of 'purple' on 'bad' faces
tweak improvements for test pretty print logging. new intersection test img files
Merge branch 'test3dexport' of github.com:openscad/openscad into test3dexport

Conflicts:
	src/OGL_helper.h
	src/renderer.cc
	tests/CMakeLists.txt
	tests/regression/openscad-camdist/camera-tests-expected.png
	tests/regression/openscad-cameyeortho/camera-tests-expected.png
	tests/regression/openscad-camrot/camera-tests-expected.png
	tests/regression/openscad-camtrans/camera-tests-expected.png
	tests/test_3d_export.py
	tests/test_cmdline_tool.py