app: brute force the session memorized size when showing a window.
The main window height was always bigger than my screen height on startup, overriding my previous session's window size. I could retrace the memorized size being changed when adding the display shell with gimp_image_window_add_shell() in gimp_display_new(), just before showing the window. Unfortunately this happens after we applied the session position/size (in gimp_image_window_session_update() at end of image window construction). I'm not sure why adding the shell increases the size of the window, especially since the window can be manually sized at the expected dimension without any graphical glitch. Maybe we could investigate this, but simply forcing any session managed window to behave as expected upon showing is not a bad move anyway and in this specific case, it works fine.
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