Fix empty region if the scale is smaller than 1

It may be used by Layer of SurfaceFlinger to calculate
touchable region. It should be able to scale a window
to a smaller size.

Bug: 111440400
Test: manual - An input-associated surface with a scaling
      matrix that has dsdx, dsdy < 1. Use "dumpsys input"
      to observe its touchable region.

Change-Id: I495bd16acec77f913fd39dcac90404eddc71cabb