Thumbnailer: limit the maximum image size

Try to limit the maximum RAM usable by the thumnailer.

• Should mitigate the CCBUG: 234030
• Patch required by Merge request 68 of kimageformats
• Tested on Debian Unstable only using the trick suggested by @meven and a lot of QDebug 😄

Requires frameworks/kcoreaddons!224 (merged) and frameworks/kcoreaddons!232 (merged)