We need to use the varying_in/out variables, the code was a little
bit too modern.
At the same time remove the identity matrix and replace it by mat4(1.0).
Note: the shader should probably go into glsl files as they are not
really generated.
* this effect is way cheaper than blur, don't cache it
* use its own atom
* also pass the matrix in the x property
* remove remnants of the cache
* do just a single pass
* get rid of config ui remnants
* a copy of the blur shader to become a copy of the background contrast effect
* contrastshader actually doing the light modification
* don't expand/shrink the area
