Fixes issue when going from an archive to a folder/image.
eg. If you were reading from a zip archive, then you dragged & dropped
a folder containing images onto the window, before the fix the zip file
wouldn't be released and its images would remain cached.
Fixed: inaccurate centering.
Center is now calculated via the frame's client size rather than the
monitor's client size. This means that the window border, menu bar and
status bar are all taken into consideration.
This fixes two pieces of behavior:
1: Previously, hiding the status or menu bar wouldn't result in the center
being recalculated, despite extra space being available. Since the bars
weren't taken into consideration, the yielded space wouldn't be factored in.
2: Previously, images of a certain height (almost big enough to warrant
scroll bars) would get hidden behind the status bar and would be "centered"
far too low.