Fix display of glyphless characters with problematic fonts

Fix display of glyphless characters with problematic fonts

* src/w32term.c (x_draw_glyph_string_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.
* src/xterm.c (x_draw_glyph_string_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.
* src/w32font.c (w32font_draw): Fix background drawing for
glyphless characters that display as acronyms or hex codes in a
box.
* src/xftfont.c (xftfont_draw): Fix background drawing for
glyphless characters that display as acronyms or hex codes in a
box.
* src/xdisp.c (produce_glyphless_glyph): Compute reasonable values
for it->ascent and it->descent when the font claims preposterously
large global values.
(FONT_TOO_HIGH): Move from here...
* src/dispextern.h (FONT_TOO_HIGH): ...to here.