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
