Fix disappearing edges in lists/scrollviews/etc.

Bug #4093871

This bug was introduced by a change that modified the way fade areas
are cleared. The previous change relied on Rect::intersect() to empty
the intersected rect when the two rects don't intersect. Unfortunately
this is not what intersect() does. The fix is rather simple and
sets the layer's bounds to empty when they don't intersect with the
clip or the viewport. This has the side effect of ignoring the layer
which is the expected result.

Change-Id: Icf0038b9a476c53f8eff7084136aba3033d093e6