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.
