Improve display of glyph matrix by dump-glyph-matrix.

Improve display of glyph matrix by dump-glyph-matrix.

 src/xdisp.c (dump_glyph): Align glyph data better.  Use "pD" instead
 of a non-portable "t" to print ptrdiff_t values.  Allow up to 9
 digits for buffer positions, before misalignment starts.  Display
 "0" for integer "object" field.
 (dump_glyph_row): Adapt the header line to changes in dump_glyph.
 Display the newline glyph more unambiguously.