add missing 'invert-rgb' compositing mode
strip newlines and whitespace in XML comparison test
add test comment in relation to https://github.com/mapnik/mapnik/commit/ecc5acbdb953e172fcc652b55ed19b8b581e2146
fix conditional support for webp/jpeg/png in blending - closes #283
Prep for dev package.
update mapnik SDK for both 2.3.x and 3.x
build fixes against mapnik master
test against mapnik 3.x/master in c++11 mode
Merge branch 'master' into vt-query-async