[gst-hybris] crashes when using playbin without setting a valid texture_id

Using gstreamer1.0-hybris (1.1.4-2ubuntu4), gst crashes inside the media compat layer when trying to use playbin without setting a valid texture_id.

To reproduce:
$ apt-get install libqgsttools-p1-touch libqt5multimedia5-touch libqt5multimedia5-touch-plugins libqt5multimediaquick-p5-touch qtdeclarative5-qtmultimedia-touch-plugin qtvideonode-plugin gstreamer1.0-plugins-ugly gstreamer1.0-plugins-bad gstreamer1.0-libav gstreamer1.0-hybris
$ wget http://people.canonical.com/~jhodapp/sintel_trailer-1080p.mp4
$ gst-launch-1.0 -v playbin uri=file:///home/phablet/Videos/sintel_trailer-1080p.mp4

If you run it with debug, you'll see the SIGSEGV happens inside gstamchybris.c:135:gst_amc_codec_configure, when it calls the media_compat layer to configure the codec:
===
err = media_codec_configure (codec->codec_delegate, format->format, stc, 0);
===

Looking further at the compat layer, this happens when calling the following code (inside hybris):
===
    ALOGE("SurfaceTextureClientHybris: %p", stch);
    ALOGE("SurfaceTextureClientHybris(singleton): %p", &_SurfaceTextureClientHybris::getInstance());
    d->media_codec->configure(aformat, &_SurfaceTextureClientHybris::getInstance(), NULL, flags);
===

And it happens because when using just playbin, nobody sets up the texture_id property, and as a side effect there's none surface_texture to use.

Ideally when texture_id is not provided, the sink itself (mirsink) would need to create and provide one, otherwise the pure playbin use case (without qtmultimedia) will be broken, affecting anyone using playbin (mediascanner, qtwebkit, etc).