Now also check previous button context, when switching. This way, if the previous one was a texture one, and become valid after the last switch (e.g. a material added to a new object), relevant texture context can still be set.
Note this commit also partially fixes [#35769] The “show texture in texture tab” button in a modifier does not show up until the user manually shows the texture in the Texture tab (full fix of this one will probably implies to always have a valid ButsContextTexture [sbuts->texuser]...).
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