glsl: improve accuracy of atan()

Our current atan()-approximation is pretty inaccurate at 1.0, so
let's try to improve the situation by doing a direct approximation
without going through atan.

This new implementation uses an 11th degree polynomial to approximate
atan in the [-1..1] range, and the following identitiy to reduce the
entire range to [-1..1]:

atan(x) = 0.5 * pi * sign(x) - atan(1.0 / x)

This range-reduction idea is taken from the paper "Fast computation
of Arctangent Functions for Embedded Applications: A Comparative
Analysis" (Ukil et al. 2011).

The polynomial that approximates atan(x) is:

x   * 0.9999793128310355 - x^3  * 0.3326756418091246 +
x^5 * 0.1938924977115610 - x^7  * 0.1173503194786851 +
x^9 * 0.0536813784310406 - x^11 * 0.0121323213173444

This polynomial was found with the following GNU Octave script:

x = linspace(0, 1);
y = atan(x);
n = [1, 3, 5, 7, 9, 11];
format long;
polyfitc(x, y, n)

The polyfitc function is not built-in, but too long to include here.
It can be downloaded from the following URL:

http://www.mathworks.com/matlabcentral/fileexchange/47851-constraint-polynomial-fit/content/polyfitc.m

This fixes the following piglit test:
shaders/glsl-const-folding-01

Signed-off-by: Erik Faye-Lund <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>

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