Fix typo in update limits code

This caused upper camera limit to not be
adjusted properly when loading old levels,
so may leave some objects out of reach.

Verified fix on mountain_trials, which had
objects beyond camera limit
Relocate grey_1 slider sprite

This was handled correctly for moving platforms,
but not using the slider sprites directly. This
would cause problems when the sliders were used
as platforms.
Add updated settings file for small hill

The image was removed as part of #256, but
this caused missing images in older levels.
Now any instances of it in levels will load
the referenced image, small_1, and show as
obsolete (red background) in the editor.
The sprite isn't included in the editor menu,
so new levels won't use it accidentally.
Merge pull request #296 from SonOfBowser/testing

Fix bugs in loading legacy levels