Do not reset ROI window when switching frames

[leofang]

Ref:
https://github.com/NSLS-II/ptycho_gui/blob/3ca5dff72b48b6b3217ce1f7832a781bdf7b6e05/nsls2ptycho/ptycho_gui.py#L1104
Just re-plotting img should be sufficient.

[leofang]

I think this is relevant: @hanfeiyan reported that when changing to another scan with the same experimental setup, it is expected to reuse the ROI configuration, but it seems we currently erase it after reload.