Memory usage stays large even after image is closed

[Cameron White]

Steps to Reproduce:
1) Open an image (a large image if possible)
2) Resize the image a few times
3) Observe the current memory usage
4) Close the image
5) Memory usage decreases slightly, but remains high

[Robert Nordan (rpvn)]

Are you thinking maybe that the canvases from before resizing and/or closing aren't properly discarded?

[Cameron White (cameronwhite91)]

Yeah. In particular, it doesn't look like BaseHistoryItem.Dispose() gets called, which would account for some of the memory usage.

[Cameron White (cameronwhite91)]

These changes reduced memory usage by about 150-200 MB in my tests: https://github.com/PintaProject/Pinta/commit/062de9fff0129368d4639ede363e94f1e2e06d78

[Cameron White (cameronwhite91)]

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).

[Cameron White (cameronwhite91)]

Marking as fix committed since memory usage is now down to a more reasonable level after closing an image.