Fix [#35741] Material shows WORLD texture_context by default.

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]...).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57497 954f8c5b-7b00-dc11-b283-0030488c597c