Only copyLayer if valid rendertarget

Bug: 27407637

Texture layers will set a GL_NONE rendertarget
until they have managed to get the real rendertarget
from SurfaceTexture. If, however, copyLayer is called
prior to any calls to updateTexImage() (for example, if
no frames have yet been produced), this will
result in trying to use a Texture with a GL_NONE rendertarget
which is bad.

Change-Id: Ib7285d799cb94ebd44f07a89f2ef2075d79ef71f