Unset the weight/slant/width in the spec when realizing a font

Between commits bf0d3f76dc (2014) and 6b1ed2f2c9 (2022),
realize_gui_face called font_load_for_lface with an empty or
partly emptied font spec, i.e. it ignored a part of its attrs
argument.  The rationale given in bug#17973, which led to
bf0d3f76dc, is not clear.  However, 6b1ed2f2c9, which passes
the full font spec to font_load_for_lface and
font_find_for_lface, leads to suboptimal font choices, for
example when the font chosen for the default face has a
weight, slant or width that is not supported by other
available fonts on the system, such as 'medium' or 'heavy'.

If these attributes are not unset here, the call to
font_list_entities in font_find_for_lface arbitrarily limits
the candidate font list to those that are perfect matches for
these attributes, which means that the scoring mechanism is
bypassed.  Note that the size attribute in spec is also unset,
in font_find_for_lface.

Also allow unsetting the other attributes, for debugging purposes.

* src/xfaces.c (realize_gui_face): Unset the weight, slant and
width of the font spec.  Fixes bug#57555 and bug#59347.
(syms_of_xfaces): New variable
'realize-gui-face-ignored-spec-attributes'.

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