Calculate depthmax after pruning nodes based on minwidth.

This avoids empty whitespace (sometimes lots) above the flames that's
caused by deep stacks that are too narrow to be seen.

Requires scanning the nodes twice, but that's offset by optimizations,
especially deleting the nodes that aren't going to be shown.
Merge pull request #17 from timbunce/imageheight

Calculate depthmax after pruning nodes based on minwidth.