memory leak opening new images

[aremmiw]

When browsing a booru or reading a CBZ file, memory usage goes up for every new image that is opened and doesn't go down. Eventually ahoviewer is killed by OOM-killer.

• problem commit is probably 32062a8, previous commits didn't have this problem.
• memory usage goes down somewhat when making a new booru search or opening a new CBZ.
  • still uses a lot of memory.
• tested on GNU/Linux, both with and without optional dependencies.

ahoviewer uses gigabytes of memory after viewing 100 high-resolution (1800x2500) images in a CBZ file.
before 32062a8 it would never go above 250M.

The same issue exists when clicking on images in a booru search.