button: move button-png.c to button/

...so that all button code is in the same place.

src/button/button-png.c

@@ -0,0 +1,91 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) Johan Malm 2023
+ */
+#define _POSIX_C_SOURCE 200809L
+#include <cairo.h>
+#include <png.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+#include <wlr/util/log.h>
+#include "buffer.h"
+#include "button/button-png.h"
+#include "common/dir.h"
+#include "labwc.h"
+#include "theme.h"
+
+/* Share with session.c:isfile() */
+static bool
+file_exists(const char *path)
+{
+	struct stat st;
+	return (!stat(path, &st));
+}
+
+/* Share with xbm.c:xbm_path() */
+static char *
+button_path(const char *filename)
+{
+	static char buffer[4096] = { 0 };
+	snprintf(buffer, sizeof(buffer), "%s/%s", theme_dir(rc.theme_name), filename);
+	return buffer;
+}
+
+/*
+ * cairo_image_surface_create_from_png() does not gracefully handle non-png
+ * files, so we verify the header before trying to read the rest of the file.
+ */
+#define PNG_BYTES_TO_CHECK (4)
+static bool
+ispng(const char *filename)
+{
+	unsigned char header[PNG_BYTES_TO_CHECK];
+	FILE *fp = fopen(filename, "rb");
+	if (!fp) {
+		return false;
+	}
+	if (fread(header, 1, PNG_BYTES_TO_CHECK, fp) != PNG_BYTES_TO_CHECK) {
+		fclose(fp);
+		return false;
+	}
+	if (png_sig_cmp(header, (png_size_t)0, PNG_BYTES_TO_CHECK)) {
+		wlr_log(WLR_ERROR, "file '%s' is not a recognised png file", filename);
+		fclose(fp);
+		return false;
+	}
+	fclose(fp);
+	return true;
+}
+
+#undef PNG_BYTES_TO_CHECK
+
+void
+png_load(const char *filename, struct lab_data_buffer **buffer)
+{
+	if (*buffer) {
+		wlr_buffer_drop(&(*buffer)->base);
+		*buffer = NULL;
+	}
+
+	char *path = button_path(filename);
+	if (!file_exists(path) || !ispng(path)) {
+		return;
+	}
+
+	cairo_surface_t *image = cairo_image_surface_create_from_png(path);
+	if (cairo_surface_status(image)) {
+		wlr_log(WLR_ERROR, "error reading png button '%s'", path);
+		cairo_surface_destroy(image);
+		return;
+	}
+	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, true);
+	cairo_t *cairo = (*buffer)->cairo;
+	cairo_set_source_surface(cairo, image, 0, 0);
+	cairo_paint_with_alpha(cairo, 1.0);
+}