texture: Remove reading the texture data back in ::unrealize()
texture: Remove reading the texture data back in ::unrealize()

Reading back the texture data in unrealize does not seem like a
desirable feature any more, clutter has evolved a lot since it was
implemented.

What's wrong with it now:

  * It takes *a lot* of time to read the data back with glReadPixel(),
  * When several textures share the same CoglTexture, the same data can
    be read back multiple times,
  * If the underlying material uses multiple texture units, only the
    first one was copied back,
  * In ClutterCairoTexture, we end up having two separate copies of the
    data,
  * GL actually manages texture memory accross system/video memory
    for us!

For all the reasons above, let's get rid of the glReadPixel() in
Texture::unrealize()

Fixes: OHB#1842

GNOME/clutter,heysion/clutter-clone,nobled/clutter,jigpu/clutter,djdeath/clutter,jigpu/clutter,spatulasnout/clutter,Distrotech/clutter,collects/clutter,collects/clutter,djdeath/clutter-android,kerrickstaley/clutter-vala,dlespiau/clutter,djdeath/clutter-multithreaded,kerrickstaley/clutter-vala,rkudiyarov/clutter_osx,spatulasnout/clutter,collects/clutter,djdeath/clutter-multithreaded,ebassi/clutter,jigpu/clutter,jigpu/clutter,jigpu/clutter,GNOME/clutter,Distrotech/clutter,kerrickstaley/clutter-vala,rkudiyarov/clutter_osx,ebassi/clutter,spatulasnout/clutter,rkudiyarov/clutter_osx,jigpu/clutter,nobled/clutter,GNOME/clutter,heysion/clutter-clone,heysion/clutter-clone,GNOME/clutter,djdeath/clutter,djdeath/clutter-android,ebassi/clutter,djdeath/clutter-android,ebassi/clutter,djdeath/clutter-multithreaded,spatulasnout/clutter,Distrotech/clutter,djdeath/clutter,ebassi/clutter,dlespiau/clutter,collects/clutter,nobled/clutter,Distrotech/clutter,dlespiau/clutter,kerrickstaley/clutter-vala,collects/clutter,Distrotech/clutter,djdeath/clutter,djdeath/clutter-multithreaded,rkudiyarov/clutter_osx,djdeath/clutter-android,djdeath/clutter,djdeath/clutter-android,Distrotech/clutter,heysion/clutter-clone,heysion/clutter-clone,djdeath/clutter,GNOME/clutter,spatulasnout/clutter