util: implement table-based + linear interpolation linear-to-srgb conversion

util: implement table-based + linear interpolation linear-to-srgb conversion

Should be much faster, seems to work in softpipe.
While here (also it's now disabled) fix up the pow factor - the former value
is what is in GL core it is however not actually accurate to fp32 standard
(as it is 1.0/2.4), and if someone would do all the accurate math there's no
reason to waste 8 mantissa bits or so...

v2: use real table generating function instead of just printing the values
(might take a bit longer as it does calculations on some 3+ million floats
but much more descriptive obviously).
Also fix up another inaccurate pow factor (this time in the python code) -
wondering where the couple one bit errors came from :-(.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Zack Rusin <zackr@vmware.com>