* Code was factorized a bit with the help of max_z
* Add an extra optimization if test is ZTST_GEQUAL and min z value is
the biggest value. Z test will always be pass.
Note: due to float rounding (23 bits mantissa vs 24 bits depth) the test
is done against 0xFF_FFFE and not 0xFF_FFFF. It is wrong but GPU will
also use float so impact will be null.