Factor out meta_actor_painting_untransformed()

The painting_untransformed() function in MetaWindowGroup is useful
elsewhere, in particular if we want to check whether we can avoid
bilinear filtering when painting a texture 1:1.

https://bugzilla.gnome.org/show_bug.cgi?id=735632
Use meta_actor_painting_untransformed() for MetaShapedTexture

The old check for using NEAREST by checking clutter_actor_is_in_clone_paint()
and meta_actor_is_untransformed (actor) doesn't work properly since
clutter_actor_is_in_clone_paint() does not look at ancestors of the
actor; it only applies to a direct clone of the actor. Using
meta_actor_painting_untransformed() allows us to check exactly what we
care about rather than using tricky approximations.

https://bugzilla.gnome.org/show_bug.cgi?id=735632
Rewrite background code

The old requirement that multiple MetaBackgroundActor objects be
layered on top of each to produce blended backgrounds resulted in
extremely inefficient drawing since the entire framebuffer had
to be read and written multiple times.

 * Replace the MetaBackground ClutterContent with a plain GObject
   that serves to hold the background parameters and prerender
   textures to be used to draw the background. It handles
   colors, gradients, and blended images, but does not handle
   vignetting

 * Add vignetting to MetaBackgroundActor directly.

 * Add MetaBackgroundImage and MetaBackgroundImageCache to allow
   multiple MetaBackground objects to share the same images

By removing the usage of ClutterContent, the following optimizations
were easy to add:

 Blending is turned off when the actor is fully opaque
 Nearest-neighbour filtering is used when drawing 1:1

The GLSL vignette code is slightly improved to use a vertex shader
snippet for computing the texture coordinate => position in actor
mapping.

https://bugzilla.gnome.org/show_bug.cgi?id=735637
MetaBackgroundActor: match total dimming if GLSL is not present

Without GLSL, we didn't apply the vignetting, which not only made the
background uniform in color, it made it much lighter. Adjust for this
and make the average brightness with the vignette effect the same
with or without GLSL.

https://bugzilla.gnome.org/show_bug.cgi?id=735637
MetaBackground: add properties to set vignette settings

Make the vignette options properties so they can be animated;
modify the function-call API for meta_background_actor_set_vignette()
to correspond more closely to the 3 properties.

https://bugzilla.gnome.org/show_bug.cgi?id=735637