repair visual tests to be able to round trip (undo bits that require 3.x support)
Work in HSL so luma can be preserved
Merge branch 'mojodna-color-comp-op' into 2.3.x
workaround old GDAL bug regarding rgba images and alpha reporting - closes #2310
update expected tests after 136e97edd4275fb - closes #2311
Merge branch '2.3.x' of github.com:mapnik/mapnik into gyp