WRaster: Properly set file's mtime in the cache when loading an image
The library maintains a cache to not reload a file that was previously
loaded. In order to still reload an image in case its file would have
changed in the meantime, the cache saves the file's modification time.
As reported by Coverity (CID #331576) the 'stat' function was not on the
execution path the first time an image is loaded, which means the cache
information is populated with junk data. This could lead to an image not
being reloaded for example.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>