shootout: make mandelbrot.go more like mandelbrot.c
shootout: make mandelbrot.go more like mandelbrot.c

Surprise! The C code is using floating point values for its counters.
Its off the critical path, but the Go code and C code are supposed to
be as similar as possible to make comparisons meaningful.

It doesn't have a significant effect.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/6260058

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