Move draw_util.c to libi3.

In order to prepare for using cairo for rendering i3 decorations,
we need to make the draw_util.c from i3bar available via libi3 such
that both i3bar and i3 can use it.

relates to #1278

include/libi3.h

@@ -20,6 +20,9 @@
 #if PANGO_SUPPORT
 #include <pango/pango.h>
 #endif
+#ifdef CAIRO_SUPPORT
+#include <cairo/cairo-xcb.h>
+#endif
 
 #define DEFAULT_DIR_MODE (S_IRWXU | S_IRGRP | S_IXGRP | S_IROTH | S_IXOTH)
 
@@ -480,3 +483,93 @@ char *get_config_path(const char *override_configpath, bool use_system_paths);
  */
 int mkdirp(const char *path, mode_t mode);
 #endif
+
+#ifdef CAIRO_SUPPORT
+/* We need to flush cairo surfaces twice to avoid an assertion bug. See #1989
+ * and https://bugs.freedesktop.org/show_bug.cgi?id=92455. */
+#define CAIRO_SURFACE_FLUSH(surface)  \
+    do {                              \
+        cairo_surface_flush(surface); \
+        cairo_surface_flush(surface); \
+    } while (0)
+#endif
+
+/* Represents a color split by color channel. */
+typedef struct color_t {
+    double red;
+    double green;
+    double blue;
+
+    /* The colorpixel we use for direct XCB calls. */
+    uint32_t colorpixel;
+} color_t;
+
+/* A wrapper grouping an XCB drawable and both a graphics context
+ * and the corresponding cairo objects representing it. */
+typedef struct surface_t {
+    /* The drawable which is being represented. */
+    xcb_drawable_t id;
+
+    /* A classic XCB graphics context. */
+    xcb_gcontext_t gc;
+
+    int width;
+    int height;
+
+#ifdef CAIRO_SUPPORT
+    /* A cairo surface representing the drawable. */
+    cairo_surface_t *surface;
+
+    /* The cairo object representing the drawable. In general,
+     * this is what one should use for any drawing operation. */
+    cairo_t *cr;
+#endif
+} surface_t;
+
+/**
+ * Initialize the surface to represent the given drawable.
+ *
+ */
+void draw_util_surface_init(surface_t *surface, xcb_drawable_t drawable, int width, int height);
+
+/**
+ * Destroys the surface.
+ *
+ */
+void draw_util_surface_free(surface_t *surface);
+
+/**
+ * Parses the given color in hex format to an internal color representation.
+ * Note that the input must begin with a hash sign, e.g., "#3fbc59".
+ *
+ */
+color_t draw_util_hex_to_color(const char *color);
+
+/**
+ * Draw the given text using libi3.
+ * This function also marks the surface dirty which is needed if other means of
+ * drawing are used. This will be the case when using XCB to draw text.
+ *
+ */
+void draw_util_text(i3String *text, surface_t *surface, color_t fg_color, color_t bg_color, int x, int y, int max_width);
+
+/**
+ * Draws a filled rectangle.
+ * This function is a convenience wrapper and takes care of flushing the
+ * surface as well as restoring the cairo state.
+ *
+ */
+void draw_util_rectangle(surface_t *surface, color_t color, double x, double y, double w, double h);
+
+/**
+ * Clears a surface with the given color.
+ *
+ */
+void draw_util_clear_surface(surface_t *surface, color_t color);
+
+/**
+ * Copies a surface onto another surface.
+ *
+ */
+void draw_util_copy_surface(surface_t *src, surface_t *dest, double src_x, double src_y,
+                            double dest_x, double dest_y, double width, double height);