Disable disc cache strategy in KPat

When using the disc cache strategy, resizing a KPat window
is horrifically slow: converting the pixmaps to PNG is quite
CPU expensive, and ends up annihilating any cache benefit.

In perf, we can see the CPU usage sky-rocketing in this case
and disabling the disc cache ends up using less CPU cycles...
2 jobs for work/ducroquet/lighter-greenblaze in 1 minute (queued for 5 seconds)