Settle with 7x7 gaussian convolution kernels, maybe slightly less
accurate than previous 9x9 but fast enough to be able to use it on i915.
About a 20% percent speed gain (again, roughly measured with
videotestsrc and glimagesink sync=false). No noticeable rendering
difference with current effects.
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.
