Close opened plots to save memory and stop matplotlib from complaining

[asagi4]

Matplotlib will complain if enough opened plots go unclosed, since apparently it keeps a reference to them somewhere forever unless the close() function is called.

/home/sd/git/ComfyUI/custom_nodes/ComfyUI_Comfyroll_CustomNodes/nodes/nodes_graphics_matplot.py:135: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`). Consider using `matplotlib.pyplot.close()`.