Ignore overviews with 0 scale in pgraster

refs #2551

Postgis raster_columns view is returning NULL values for raster
overviews with large scale factors. That issue in postgis is described
in http://trac.osgeo.org/postgis/ticket/3006

This causes two main problems:

  - The first overview with scale = NULL is wrongly chosen for rendering
    always

  - The messed up scaling factor causes the render symbolizer to spent
    an insane amount of CPU and memory to render a messed up tiles.

The patch in postgis is expected to be released with the new version, a
few months from now.

Conflicts:
	plugins/input/pgraster/pgraster_datasource.cpp
use font-feature-settings per text_item
update visual test - font-feature-settings
Merge pull request #2536 from mapycz/improve-font-feature-settings

font-feature-settings improvements
stop setting -Wsign-compare since this is too noisy as a default
Merge branch 'master' of github.com:mapnik/mapnik into image_data_any