add support for applying custom comp_op and opacity when blending map background image into background color at render time - refs mapnik/mapnik-support#8
further develop 2.3.0 changelog
more changelog additions
more 2.3.x changelog updates
Merge pull request #1966 from mapnik/background-image-comp-op

Map background-image-comp-op and background-image-opacity
add mention of background-image-opacity and background-image-comp-op to changelog - refs #1966
Merge branch 'master' of github.com:mapnik/mapnik
[mappy] Parallel feature requests supports - PostGIS async requests