SVG font input empty chars not output to SFD or fonts

Magic flag `widthset` was not being set for input glyphs upon parsing
from SVG font files.  This flag is used by `sfd.c` routine `SFDOmit()`
to judge whether a character was significant enough to be output.

Upon first input from SVG font file, empty glyphs (not containing any
drawing instructions / spline info) would appear in character view
(as X'd boxes).  But these would not be output to a new SFD file or
any generated fonts.  Restarting FF using the previous SFD file as
input would find none of the empty characters - they'd disappeared.

This would affect such glyphs as `'.null'`, `'space'`, and maybe
`'nomarkingreturn'`.

Fix adds one line to set flag into `svg.c` routine `SVGParseGlyphArgs()`.