swaybar/tray: replace icon lookup with libsfdo

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

swaybar/tray/tray.c

@@ -1,11 +1,12 @@
 #include <cairo.h>
 #include <poll.h>
+#include <sfdo-basedir.h>
+#include <sfdo-icon.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
 #include "swaybar/config.h"
 #include "swaybar/bar.h"
-#include "swaybar/tray/icon.h"
 #include "swaybar/tray/host.h"
 #include "swaybar/tray/item.h"
 #include "swaybar/tray/tray.h"
@@ -34,6 +35,22 @@ static int handle_lost_watcher(sd_bus_message *msg,
 	return 0;
 }
 
+static struct sfdo_icon_theme *load_icon_theme(struct sfdo_icon_ctx *ctx,
+		const char *name) {
+	int options = SFDO_ICON_THEME_LOAD_OPTION_ALLOW_MISSING |
+		SFDO_ICON_THEME_LOAD_OPTION_RELAXED;
+	struct sfdo_icon_theme *theme = sfdo_icon_theme_load(ctx, name, options);
+	if (!theme) {
+		// _ALLOW_MISSING falls back to hicolor when the named theme is
+		// missing, but returns NULL when the theme is found but invalid.
+		// Manually retry with hicolor in that case.
+		sway_log(SWAY_DEBUG, "Failed to load icon theme '%s', "
+				"falling back to hicolor", name ? name : "(default)");
+		theme = sfdo_icon_theme_load(ctx, "hicolor", options);
+	}
+	return theme;
+}
+
 struct swaybar_tray *create_tray(struct swaybar *bar) {
 	sway_log(SWAY_DEBUG, "Initializing tray");
 
@@ -46,12 +63,29 @@ struct swaybar_tray *create_tray(struct swaybar *bar) {
 
 	struct swaybar_tray *tray = calloc(1, sizeof(struct swaybar_tray));
 	if (!tray) {
+		sd_bus_flush_close_unref(bus);
 		return NULL;
 	}
 	tray->bar = bar;
 	tray->bus = bus;
 	tray->fd = sd_bus_get_fd(tray->bus);
 
+	struct sfdo_basedir_ctx *basedir_ctx = sfdo_basedir_ctx_create();
+	if (!basedir_ctx) {
+		sway_log(SWAY_ERROR, "Failed to create sfdo basedir context");
+		goto error;
+	}
+	tray->icon_ctx = sfdo_icon_ctx_create(basedir_ctx);
+	sfdo_basedir_ctx_destroy(basedir_ctx);
+	if (!tray->icon_ctx) {
+		sway_log(SWAY_ERROR, "Failed to create sfdo icon context");
+		goto error;
+	}
+
+	const char *theme_name = bar->config->icon_theme;
+	tray->icon_theme = load_icon_theme(tray->icon_ctx, theme_name);
+	tray->icon_theme_name = theme_name ? strdup(theme_name) : NULL;
+
 	tray->watcher_xdg = create_watcher("freedesktop", tray->bus);
 	tray->watcher_kde = create_watcher("kde", tray->bus);
 
@@ -68,9 +102,13 @@ struct swaybar_tray *create_tray(struct swaybar *bar) {
 	init_host(&tray->host_xdg, "freedesktop", tray);
 	init_host(&tray->host_kde, "kde", tray);
 
-	init_themes(&tray->themes, &tray->basedirs);
-
 	return tray;
+
+error:
+	sfdo_icon_ctx_destroy(tray->icon_ctx);
+	sd_bus_flush_close_unref(tray->bus);
+	free(tray);
+	return NULL;
 }
 
 void destroy_tray(struct swaybar_tray *tray) {
@@ -86,10 +124,37 @@ void destroy_tray(struct swaybar_tray *tray) {
 	destroy_watcher(tray->watcher_xdg);
 	destroy_watcher(tray->watcher_kde);
 	sd_bus_flush_close_unref(tray->bus);
-	finish_themes(tray->themes, tray->basedirs);
+	sfdo_icon_theme_destroy(tray->icon_theme);
+	sfdo_icon_ctx_destroy(tray->icon_ctx);
+	free(tray->icon_theme_name);
 	free(tray);
 }
 
+void tray_reload_icon_theme(struct swaybar_tray *tray, const char *name) {
+	if ((!tray->icon_theme_name && !name) ||
+			(tray->icon_theme_name && name &&
+				strcmp(tray->icon_theme_name, name) == 0)) {
+		return;
+	}
+	sway_log(SWAY_DEBUG, "Reloading tray icon theme: '%s' -> '%s'",
+			tray->icon_theme_name ? tray->icon_theme_name : "(default)",
+			name ? name : "(default)");
+	sfdo_icon_theme_destroy(tray->icon_theme);
+	tray->icon_theme = load_icon_theme(tray->icon_ctx, name);
+	free(tray->icon_theme_name);
+	tray->icon_theme_name = name ? strdup(name) : NULL;
+
+	// invalidate per-SNI cached state so icons reload on next render
+	for (int i = 0; i < tray->items->length; ++i) {
+		struct swaybar_sni *sni = tray->items->items[i];
+		sfdo_icon_theme_destroy(sni->icon_theme_override);
+		sni->icon_theme_override = NULL;
+		sni->icon_size = 0;
+		sni->target_size = 0;
+	}
+	set_bar_dirty(tray->bar);
+}
+
 void tray_in(int fd, short mask, void *data) {
 	struct swaybar *bar = data;
 	int ret;