Ensure read-only transactions

Closes #2142
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
return code 0 to gracefully exit if python bindings are not available
scons: fix 'make clean' so it actually runs completely
Revert "return code 0 to gracefully exit if python bindings are not available"

This reverts commit 540f70a1f4baf6fe06b3c1c52d99a28a44945520.
fix -Wsign-compare warnings in several grammars
Merge pull request #2304 from strk/2.3.x-readonly

Ensure read-only transactions
scons: fix first-run clean problem #2112
Merge branch '2.3.x' of github.com:mapnik/mapnik into 2.3.x
Implement input plugin for PostGIS Rasters

Closes #1660.
Includes python bindings and testcases.
remove deprecation warnings in 2.3.x - refs #1340
scons: no need to link to fontconfig in static cairo build
travis: attempt to fix gdal install
Fix false negative responses from python tests

Closes #2298
Merge pull request #2343 from strk/2.3.x-false-negatives

Fix false negative responses from python tests
Add optional extent paramater to ogr plugin
travis: only test with clang