Use less memory when loading camera previews: Refs #2959
Use less memory when loading camera previews: Refs #2959

Reduces memory usage by loading images at scaled size when preview
is unavailable.

abrauchli/shotwell,jimbonator/shotwell,abrauchli/shotwell,jimbonator/shotwell,abrauchli/shotwell,ritchiewilson/shotwell,jimbonator/shotwell,ritchiewilson/shotwell,ritchiewilson/shotwell