Complete fix for [#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 (and better fix for [#35741] Material shows WORLD texture_context by default).

Simply recompute texture context on every redraw, like already done for sbuts->texuser (also called texture context :/ ).

Also allows simplification, buttons_check_texture_context can be merged back into set_texture_context...

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57500 954f8c5b-7b00-dc11-b283-0030488c597c