label-freetype: implement bold font handling

label-freetype: implement bold font handling

Bold monospace fonts are prominently used in the console output printed
by systemd. Without them, the output looks incorrect compared to pango.
This still is far from a complete implementation of rich text, but
brings label-freetype much closer to what label-pango does for the
default theme.

This commit introduces a glyph_face variable in load_glyphs() that holds
the correct face for the current glyph (label->face or
label->bold_face). This face is then passed on to load_glyph() and
finish_measuring_line() to ensure correct font measurements.