Generate a normalized gaussian kernel with given size and standard
deviation on the fly.
Remove "norm_const" uniform from convolution shaders and provide a
normalized kernel instead. Remove norm_offset uniform as it was always
zero, will reintroduce it if really needed in the future. Thanks to Eric
Anholt for suggesting it.
Save some ALU instruction calculating directly the coordinate for
texture lookup instead of summing an offset.
Still exceed maximum indirect texture lookups on i915, the only solution
I see is using a 3x3 kernel.
For now only identity, mirror and squeeze effects are available.
Maybe some factorization is needed about compilation shader
before to put the other effects since only a copy/past is needed,
at least until effect number 9: heat.
The effects from 10:sepia to 15:glow require more work.
