LoadCacheImage: Fix check of filedate causing unnecessary cache reload

This bug causes premature discarding of files from the cache and is
responsible for flicker in MythGallery while building the thumbnail
preview images for a folder containing a large (>30) number of files.

Signed-off-by: Lawrence Rust <lvr@softsystem.co.uk>
Signed-off-by: Stuart Morgan <smorgan@mythtv.org>