ssd: allow arbitrary cairo pattern as titlebar background

The titlebar background is now first rendered to a 1px wide buffer,
then stretched horizontally. This allows vertical gradients to be used.

include/theme.h

@@ -8,7 +8,7 @@
 #ifndef LABWC_THEME_H
 #define LABWC_THEME_H
 
-#include <stdio.h>
+#include <cairo.h>
 #include <wlr/render/wlr_renderer.h>
 #include "ssd.h"
 
@@ -89,6 +89,18 @@ struct theme {
 		struct lab_img *button_imgs
 			[LAB_SSD_BUTTON_LAST + 1][LAB_BS_ALL + 1];
 
+		/*
+		 * The titlebar background is specified as a cairo_pattern
+		 * and then also rendered into a 1px wide buffer, which is
+		 * stretched horizontally across the titlebar.
+		 *
+		 * This approach enables vertical gradients while saving
+		 * some memory vs. rendering the entire titlebar into an
+		 * image. It does not work for horizontal gradients.
+		 */
+		cairo_pattern_t *titlebar_pattern;
+		struct lab_data_buffer *titlebar_fill;
+
 		struct lab_data_buffer *corner_top_left_normal;
 		struct lab_data_buffer *corner_top_right_normal;