Hotfix alpha settings_overwrite["mpl_transparent_background"] = False

Explanation of changes

Actively set the alpha of figure for mpl_transparent_background=False

@AdriaanRol another one ASAP please

Motivation of changes

Passing a settings_overwrite={"mpl_transparent_background": False} to the analysis does not make the background white... This fixes it.

Same applies for base_analysis.setting["mpl_transparent_background"] = False

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