[ftview] New key 'Z' to flip SVG rendering.

* src/ftcommon.h (FTDemo_Handle): New field `use_svg`.
* src/ftcommon.c (FTDemo_New, FTDemo_Update_Current_Flags): Handle it.

* src/ftview.c (Process_Event): Make key 'Z' toggle SVG usage.
(event_help, write_header): Updated.