set correct crop rectangle in LayerBase::setCrop

The crop always had left=top=0, because the crop position
and the layer's transform were merged together in
computeBounds() (which really used to compute the
bounds in screen space, which we usually call the
"frame" elsewhere in the code)

Note: in practice this crop value is not used by
hwc, because it's overridden in Layer::setGeometry(), which
is why this bug was never apparent.

Change-Id: I1ec6400a8fc8314408e4252708f43ea98c2fe64e