Extract cairo_clear_surface.

This commit refactors the i3bar drawing code to also define an abstraction
function for clearing a surface. This is needed to fully abstract i3bar/xcb.c's
drawing code so that we can introduce a switch to easily exchange the
underlying drawing mechanism.

i3bar/include/cairo_util.h

@@ -89,6 +89,13 @@ void cairo_draw_text(i3String *text, surface_t *surface, color_t fg_color, color
  */
 void cairo_draw_rectangle(surface_t *surface, color_t color, double x, double y, double w, double h);
 
+/**
+ * Clears a surface with the given color.
+ * Note that the drawing is done using CAIRO_OPERATOR_SOURCE.
+ *
+ */
+void cairo_clear_surface(surface_t *surface, color_t color);
+
 /**
  * Copies a surface onto another surface.
  * Note that the drawing is done using CAIRO_OPERATOR_SOURCE.