Big optimization: Properly check for points in the cardioid
Big optimization: Properly check for points in the cardioid

After some tweaking of mandelbrot_in_cardioid I've come up
with the proper polar equation and reasonable upper bounds to
avoid checking points that are definitely outside of the
cardioid.

Signed-off-by: Fernando J. Pereda <adf84623333d92dbb18c359aa6fbf8ee09ba3057@gentoo.org>

fpereda/gmandel,fpereda/gmandel