Don't destroy the same texture twice

Bug #7221449

SurfaceTexture already deletes the GL texture when detachFromContext
is invoked. The newly introduced refcount would casue the Layer
object to be destroyed later and attempt to delete the GL texture
again. By the time the second cleanup occurs, the texture name
might have been reused by somebody else, resulting in erroneous
behaviors.

Change-Id: I257c589fea64b34c00f46fbfaa7732e6854a5e41