Reduce memory usage when caching image-buffers

Remove invalidated caches and limit number of active cache entries.

Ref Issue #58
2 jobs for !6 with pixbuf_caching in 1 minute and 44 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Build
passed #1359
build

00:00:47

 
  Distcheck
passed #1360
distcheck

00:00:56