Fix regression with hardware layers.

The renderer was checking whether the layer's region is a rect to
draw it with a simple quad. This test was done against the layer's
region structure, which was cleared after generating the mesh.
Unfortunately Region::isRect() returns true when the region is
empty, which was causing the renderer to always use a quad to
draw a mesh on screen.

Change-Id: If4b874579ee0cedcb67367c9efa5c0c98ca55231