bugfix: use the imlib cache for every imlib context used
bugfix: use the imlib cache for every imlib context used

calling imlib_set_cache_size() before a context is created by fluxbox creates
an 'unknown' context. that one is never freed at shutdown.

olivergondza/fluxbox,naimdjon/fluxbox,luebking/fluxbox,Arkq/fluxbox,olivergondza/fluxbox,luebking/fluxbox,olivergondza/fluxbox,olivergondza/fluxbox,lukoko/fluxbox,luebking/fluxbox,naimdjon/fluxbox,luebking/fluxbox,lukoko/fluxbox,lukoko/fluxbox,Arkq/fluxbox,luebking/fluxbox,naimdjon/fluxbox,olivergondza/fluxbox,Arkq/fluxbox,naimdjon/fluxbox,Arkq/fluxbox,lukoko/fluxbox,naimdjon/fluxbox,lukoko/fluxbox,Arkq/fluxbox