This is a moderately fast two-pass gaussian blur
implementation. It downscales the framebuffer by
half before applying the gaussian shader, which
cuts down rendering time quite considerably.
The blur shader takes 3 uniforms as input: the blur
radius; whether to blur vertically or horizontally;
and a brightness value.
The blur radius is treated as an integer in C land
to simplify calculations. The vertical parameter is
treated as an integer by the shader simply due to
Cogl not having proper boolean support in snippets.
At last, brightness is also added to avoid needing
to use an extra effect to achieve that.
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