Convert the ASCII art documentation in cogl-vertex-buffer to PNGs

The documentation for cogl_vertex_buffer_indices_get_for_quads was
using ugly ASCII art to draw the diagrams. These have now been
replaced with PNG figures.

cogl/cogl-vertex-buffer.h

@@ -421,27 +421,20 @@ cogl_vertex_buffer_unref (CoglHandle handle) G_GNUC_DEPRECATED;
  *
  * 0, 1, 2, 0, 2, 3, 4, 5, 6, 4, 6, 7 ... etc
  *
- * For example, if you submit vertices for a quad like this:
+ * For example, if you submit vertices for a quad like like that shown
+ * in <xref linkend="quad-indices-order"/> then you can request 6
+ * indices to render two triangles like those shown in <xref
+ * linkend="quad-indices-triangles"/>.
  *
- * |[
- *    0        3
- *     ########
- *     #      #
- *     #      #
- *     ########
- *    1        2
- * ]|
+ * <figure id="quad-indices-order">
+ *   <title>Example of vertices submitted to form a quad</title>
+ *   <graphic fileref="quad-indices-order.png" format="PNG"/>
+ * </figure>
  *
- * Then you can request 6 indices to render two triangles like this:
- *
- * |[
- *    0           0        3
- *     ##          ########
- *     # ##          ##   #
- *     #   ##          ## #
- *     ########          ##
- *    1        2           2
- * ]|
+ * <figure id="quad-indices-triangles">
+ *   <title>Illustration of the triangle indices that will be generated</title>
+ *   <graphic fileref="quad-indices-triangles.png" format="PNG"/>
+ * </figure>
  *
  * Returns: A %CoglHandle containing the indices. The handled is
  * owned by Cogl and should not be modified or unref'd.

doc/reference/cogl/Makefile.am

@@ -94,6 +94,8 @@ EXTRA_HFILES=
 HTML_IMAGES = \
 	fill-rule-non-zero.png \
 	fill-rule-even-odd.png \
+	quad-indices-order.png \
+	quad-indices-triangles.png \
 	cogl_ortho.png
 
 # Extra SGML files that are included by $(DOC_MAIN_SGML_FILE).
@@ -125,4 +127,6 @@ include $(top_srcdir)/gtk-doc.make
 EXTRA_DIST += \
 	fill-rule-non-zero.png \
 	fill-rule-even-odd.png \
+	quad-indices-triangles.png \
+	quad-indices-order.png \
 	cogl_ortho.png