Drops the in-tree icon theme parser and lookup logic in favor of
libsfdo-icon, which implements the icon theme spec.
Absolute icon paths are handled in swaybar itself with an access()
probe, since libsfdo intentionally doesn't deal with them. Pixmap
fallback still kicks in if load_image() fails on the absolute path.
KDE's IconThemePath is honored via sfdo_icon_theme_load_from() with
a per-SNI override theme that gets dropped when the property changes.
The min/max size cache per SNI is replaced by re-lookup when the
rendered size changes; libsfdo caches the parsed theme internally
and reads gtk's icon-theme.cache files, so this is cheap.
Theme switching at runtime is preserved via a new
tray_reload_icon_theme() called from parse_bar_config(). The old
code worked here implicitly because all themes were preloaded.
Closes: https://github.com/swaywm/sway/issues/8607
New warnings can be hard to notice in CI, since CI will just pass in
that case. Meson sometimes uses warnings for important mistakes, e.g.
invalid option.
Let's turn warnings into errors so that we can spot these more easily.
