Cache flush interval for images not working as expected

[kmarius]

Issue

The flush interval given with the -f flag in an ${image} does not work as expected. The offending line is

https://github.com/brndnmtthws/conky/blob/8f5014b90f1bc9f999beff752a9b369e4885f0d6/src/imlib2.cc#L219

If update_interval is 5 and flush_interval is 4 the image will be reloaded every 20 seconds. If both are 2 and conky is started on an odd second, then the current time in cur will always be odd. Flushes are only guaranteed if gcd(update_interval, flush_interval) == 1.