allow alternate image comparison tool diffpng based on Hector Yee's algorithm

Conflicts:
	tests/CMakeLists.txt
	tests/test_cmdline_tool.py
test ImageMagick to see if two non-identical images are considered identical.
on failure, fall back to diffpng comparison (diffpng.cpp)
fix bug in imagemagick comparison detector
fallback to diffpng if ImageMagick is missing
allow alternate image comparison tool diffpng based on Hector Yee's algorithm

Conflicts:
	tests/CMakeLists.txt
update diffpng

Conflicts:
	tests/CMakeLists.txt
update diffpng.cpp
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

Conflicts:
	src/OGL_helper.h
	tests/CMakeLists.txt
	tests/test_3d_export.py
	tests/test_cmdline_tool.py
	tests/test_pretty_print.py
tweak improvements for test pretty print logging. new intersection test img files

Conflicts:
	tests/regression/opencsgtest/intersection2-tests-expected.png
	tests/test_pretty_print.py
debugging messages. update diffpng

Conflicts:
	src/Camera.cc
	tests/CMakeLists.txt
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

Conflicts:
	src/OGL_helper.h
	tests/CMakeLists.txt
	tests/diffpng.cpp
	tests/test_3d_export.py
	tests/test_pretty_print.py
re-doing the test generation for 3d formats that dont preserve color
turn off pretty-print debugging
Use --render when creating expected images
fix bug where --render was not using proper colorscheme due to
getFacetColor in OGL_helper.h not being overridden in CGAL_renderer.h

Conflicts:
	src/CGALRenderer.cc
	src/CGAL_renderer.h
	src/OGL_helper.h
	src/renderer.cc
	tests/CMakeLists.txt
	tests/export_import_pngtest.py
	tests/regression/stlpngtest/demo_cut-expected.png
	tests/regression/stlpngtest/difference-expected.png
	tests/regression/stlpngtest/fence-expected.png
	tests/regression/stlpngtest/rounded_box-expected.png
	tests/regression/stlpngtest/surface-expected.png
	tests/regression/stlpngtest/translation-expected.png
new tests for stlpngtest offpngtest

Conflicts:
	tests/regression/offpngtest/demo_cut-expected.png
	tests/regression/offpngtest/difference-expected.png
	tests/regression/offpngtest/fence-expected.png
	tests/regression/offpngtest/rounded_box-expected.png
	tests/regression/offpngtest/surface-expected.png
	tests/regression/offpngtest/translation-expected.png
Tag all example tests correctly
run the diffpng image comparer without creating an output image, for speed
Synced some remaining fixes from the test_3d_export branch
add more expected images. remove accidentally added intermediate files.

Conflicts:
	tests/regression/offpngtest/demo_cut-expected.png
	tests/regression/offpngtest/difference-expected.png
	tests/regression/offpngtest/fence-expected.png
	tests/regression/offpngtest/rounded_box-expected.png
	tests/regression/offpngtest/surface-expected.png
	tests/regression/offpngtest/translation-expected.png
	tests/regression/stlpngtest/demo_cut-expected.png
	tests/regression/stlpngtest/difference-expected.png
	tests/regression/stlpngtest/fence-expected.png
	tests/regression/stlpngtest/rounded_box-expected.png
	tests/regression/stlpngtest/surface-expected.png
	tests/regression/stlpngtest/translation-expected.png