Blender Git Loki

- fixed switching contexts... the previous commit about this was too
rigurous. Now it only switches context when selecting a new object
type, also "invisble", when shading buttons are not drawn.

What I committed changed context always when pressing the 'shading
context' icon button. This was annoying when you were editing textures
or radiosity, world, etc. In such cases it should just draw the old
settings.