Bind latest buffer when HwcComposer2D retrieves it

Make sure when HwcComposer2D gets the latest ImageLayer buffer that its also
bound to the OpenGL texture. Otherwise the texture's buffer will be
held indefinitely as long as all rendering is done by HwcComposer2D

Change-Id: I06a621a4f5a0248529465a7a99ef1c4869579ec7