Revert scale3.

As I pointed out at the time, this is entirely redundant. A scale 3 background is a scale 9
background with two of its edges resized to 0.

  Scale3Background.Horizontal = Scale9Background with yaxis.resize(0, 0) and (2, 0)
  Scale3Background.Vertical = Scale9Background with xaxis.resize(0, 0) and (2, 0)
New features to facilitate erstwhile "scale 3" behavior.

This also introduces the leaf type return value for Background.scale9.  Now you can write:

    Style.BACKGROUND.is(Background.scale9(image).corners(10).inset(5))

I think this would make sense for the other methods too, but I'm being conservative here.
Unit tests for Scale9.

These are really testing Scale9.Axis but that's the only meaty bit.
Fix bug found by unit tests!
Make this float math more uniform.

Also found by unit test, but not strictly a failure.
Revert "Depend on demo assets for ios build."

This reverts commit c3a73ff72668e1c0b1b268ba3e6ad4adfd66b346.

As Michael pointed out, there is no reason to actually build the assets for ios, so no dependency
is needed.