Blender Git Loki

* overlay texture now only updates when the texture has changed
* overlay does not delete/recreate the texture slot, but uses glTexSubImage to update the existing texture
* overlay texture now has alpha so that blacker areas are also more transparent
* overlay texture size increased to 512x512
* added a 'changed_timestamp' field to the texture preview structure to allow notification if the preview has changed to be monitored by more parts of the program than just the icon renderer
* All of this is the first step in unifying the overlay with a new texcache
* Also first steps in making the overlay a seperate operator