#247 gerbv max zoom range reduced compared to previous version

[Marissa Giustina]

In a previous version, it was possible to zoom in significantly further than it is now. I work with very small feature sizes, so the current zoom limit renders gerbv inadequate for me.

I found this exchange from a few years ago: https://gerbv-devel.narkive.com/7UhnyfJk/latest-gerbv-zoom-range-limited
Accordingly I modified GERBV_SCALE_MAX in gerbv.h from 3000 to 20000. Unfortunately, this did not change the maximum zoom I can achieve.