Support svg buttons

In the theme directory add close-{active,inactive}.svg instead of
close.xbm - and similarly for iconify, menu and max.

src/button/button-svg.c

@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) Johan Malm 2023
+ */
+#define _POSIX_C_SOURCE 200809L
+#include <cairo.h>
+#include <librsvg/rsvg.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <wlr/util/log.h>
+#include "buffer.h"
+#include "button/button-svg.h"
+#include "button/common.h"
+#include "common/file-helpers.h"
+#include "labwc.h"
+#include "theme.h"
+
+void
+button_svg_load(const char *button_name, struct lab_data_buffer **buffer,
+		int size)
+{
+	if (*buffer) {
+		wlr_buffer_drop(&(*buffer)->base);
+		*buffer = NULL;
+	}
+
+	char filename[4096] = { 0 };
+	button_filename(button_name, filename, sizeof(filename));
+
+	GError *err = NULL;
+	RsvgRectangle viewport = { .width = size, .height = size };
+	RsvgHandle *svg = rsvg_handle_new_from_file(filename, &err);
+	if (err) {
+		wlr_log(WLR_DEBUG, "error reading svg %s-%s\n", filename, err->message);
+		g_error_free(err);
+		/*
+		 * rsvg_handle_new_from_file() returns NULL if an error occurs,
+		 * so there is no need to free svg here.
+		 */
+		return;
+	}
+
+	cairo_surface_t *image = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, size, size);
+	cairo_t *cr = cairo_create(image);
+
+	rsvg_handle_render_document(svg, cr, &viewport, &err);
+	if (err) {
+		wlr_log(WLR_ERROR, "error rendering svg %s-%s\n", filename, err->message);
+		g_error_free(err);
+		goto error;
+	}
+
+	if (cairo_surface_status(image)) {
+		wlr_log(WLR_ERROR, "error reading svg button '%s'", filename);
+		goto error;
+	}
+	cairo_surface_flush(image);
+
+	double w = cairo_image_surface_get_width(image);
+	double h = cairo_image_surface_get_height(image);
+	*buffer = buffer_create_cairo((int)w, (int)h, 1.0, /* free_on_destroy */ true);
+	cairo_t *cairo = (*buffer)->cairo;
+	cairo_set_source_surface(cairo, image, 0, 0);
+	cairo_paint_with_alpha(cairo, 1.0);
+
+error:
+	cairo_destroy(cr);
+	cairo_surface_destroy(image);
+	g_object_unref(svg);
+}