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.