The destructor for TextureRefHolder does not handle a lost context

[Chris Coulson]

The destructor for TextureRefHolder calls GLES2Decoder::MakeCurrent() before dropping its reference to TextureRef, but MakeCurrent() will fail if the GL context is lost. In this case, bad things will happen if TextureRef is the last reference to the underlying Texture, as TextureManager assumes the caller has made the context current and attempts to delete the texture