In tachyon 0.99.2 the keyword `focallength` was changed to `focaldist`.
To support it, when running on version >= 0.99.2 we "patch" the model as
constructed by class `sage.plot.plot3d.tachyon.Tachyon`.

In the future (possibly when tachyon in sage gets upgraded), all the
focallength occurences in sage.plot.plot3d.tachyon can be replaced by
focaldist for consistency with new tachyon, and the logic here can be
reversed (i.e. patch the model when self.version() < '0.99.2') or just
drop support for old versions.

Debian patches tachyon so it won't report the version.

We hardcode '0.99' since that's the version they ship.