NS equivalents of xterm.c and w32term.c changes

NS equivalents of xterm.c and w32term.c changes

* src/nsterm.m (ns_maybe_dumpglyphs_background): Force redraw of
glyph string background also when the font in use claims
preposterously large global height value.  Helps to remove
artifacts left from previous displays when glyphless characters
are displayed as hex code in a box.
(x_new_font): Call get_font_ascent_descent to obtain a reasonable
value for FRAME_LINE_HEIGHT, even when a font claims very large
value for its height.