ssd: support window icons

The default `titleLayout` is updated to `icon:iconify,max,close` which
replaces the window menu button with the window icon.

When the icon file is not found or could not be loaded, the window menu
icon as before is shown.

The icon theme can be selected with `<theme><icon>`.

This commit adds libsfdo as an optional dependency. `-Dicon=disabled` can
be passsed to `meson setup` command in order to disable window icon, in
which case the window icon is always replaced with a window menu button.

include/img/img-png.h

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_IMG_PNG_H
+#define LABWC_IMG_PNG_H
+
+struct lab_data_buffer;
+
+void img_png_load(const char *filename, struct lab_data_buffer **buffer);
+
+#endif /* LABWC_IMG_PNG_H */

include/img/img-svg.h

@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_IMG_SVG_H
+#define LABWC_IMG_SVG_H
+
+struct lab_data_buffer;
+
+void img_svg_load(const char *filename, struct lab_data_buffer **buffer,
+	int size);
+
+#endif /* LABWC_IMG_SVG_H */

include/img/img-xbm.h

@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_IMG_XBM_H
+#define LABWC_IMG_XBM_H
+
+struct lab_data_buffer;
+
+/**
+ * img_xbm_from_bitmap() - create button from monochrome bitmap
+ * @bitmap: bitmap data array in hexadecimal xbm format
+ * @buffer: cairo-surface-buffer to create
+ * @rgba: color
+ *
+ * Example bitmap: char button[6] = { 0x3f, 0x3f, 0x21, 0x21, 0x21, 0x3f };
+ */
+void img_xbm_from_bitmap(const char *bitmap, struct lab_data_buffer **buffer,
+	float *rgba);
+
+/* img_xbm_load - Convert xbm file to buffer with cairo surface */
+void img_xbm_load(const char *filename, struct lab_data_buffer **buffer,
+	float *rgba);
+
+#endif /* LABWC_IMG_XBM_H */