Compiling cell.hh with clang generates lots of warnings of the form:

./cell.hh:102:9: warning: taking address of packed member 'm_colors'
of class or structure 'VteCellAttr' may result in an unaligned pointer
value [-Waddress-of-packed-member]
        CELL_ATTR_COLOR(fore)
        ^~~~~~~~~~~~~~~~~~~~~
./cell.hh:94:46: note: expanded from macro 'CELL_ATTR_COLOR'
                vte_color_triple_set_##name(&m_colors, value); \
                                             ^~~~~~~~

However, VteCellAttr is only ever used as a member of VteCell,
where it has offset 4, resulting in the pointers being aligned
sufficiently.