pixman-combine-float.c: Use IS_ZERO() in clip_color() and set_sat()

pixman-combine-float.c: Use IS_ZERO() in clip_color() and set_sat()

The clip_color() function has some checks to avoid division by zero,
but they are done by comparing the value to 4 * FLT_EPSILON, where a
better choice is the IS_ZERO() macro that compares to +/- FLT_MIN.

In set_sat(), the check is that *max > *min before dividing by *max -
*min, but that has the potential problem that interactions between GCC
optimizions and 80 bit x87 registers could mean that (*max > *min) is
true in 80 bits, but (*max - *min) is 0 in 32 bits, so that the
division by zero is not prevented. Using IS_ZERO() here as well
prevents this.