#3 Replace hard-coded math constants with cmath constants

[Rob Gill]

agg_trans_affine.h uses hard-coded math constants for 1/sqrt(2), a more precise value for this constant if provided by cmath's M_SQRT1_2.

Proposed patch included.

[John Horigan]

M_SQRT1_2 is from the POSIX standard, not any C or C++ standard. I can update the file with a more precise version of the constant.

[Jim Barry]

Addressed in r136.