Removed sine curve cooling; not worth the computational weight, and I'm not
entirely sure it was working correctly anyway.  Removed Fire._toRad.

Added Fire.intensity: sets what index in the color palette is used to
fill hotspots (rather than just using the last index).  Values are 0..100.

In Fire constructor, made sure heat can never be higher than max_x.

In interpolate_point, added early returns if the resulting color is 0.
Set heat back down to 32.