Comment 4 for bug 987646

Fixed an issue with image previews in the Open Images pad not being disposed: https://github.com/PintaProject/Pinta/commit/5cde8074c2f51ef44724588ed1daacddad82c41d

In my test case, Pinta's memory usage now returns from around 415MB to around 120MB after closing my test image (previously it only returned to ~390MB).