render: Use "long long" instead of "double" for arithmetic.

render: Use "long long" instead of "double" for arithmetic.

When I wrote this code in 2010, it seemed reasonable to me to use
floating-point arithmetic to do large integer calculations, because 64-bit
machines were not yet pervasive.  Over 8 years later, 32-bit machines are
now a rarity.  This commit switches types and modernizes the coding style.

This fixed an assertion failure that I was getting mysteriously while
rendering a particular table.  I didn't investigate further because using
integers seemed like a much better idea anyway.