A `PangoGlyphItemIter` is an iterator over the clusters in a `PangoGlyphItem`.
The *forward direction* of the iterator is the logical direction of text. That is, with increasing start_index and
start_char values. If glyph_item is right-to-left (that is, if `glyph_item->item->analysis.level` is odd),
then start_glyph decreases as the iterator moves forward. Moreover, in right-to-left cases, start_glyph is
greater than end_glyph.
An iterator should be initialized using either init_start or init_end, for forward and backward iteration respectively, and walked over using any desired mixture of next_cluster and prev_cluster.
A common idiom for doing a forward iteration over the clusters is:
``` PangoGlyphItemIter cluster_iter; gboolean have_cluster;
for (have_cluster = pango_glyph_item_iter_init_start (&cluster_iter, glyph_item, text); have_cluster; have_cluster = pango_glyph_item_iter_next_cluster (&cluster_iter)) { ... } ```
Note that text is the start of the text for layout, which is then indexed by `glyph_item->item->offset` to get to the
text of glyph_item. The start_index and end_index values can directly index into text.
The start_glyph, end_glyph, start_char, and end_char values however are zero-based
for the glyph_item. For each cluster, the item pointed at by the start variables is included in the cluster while the one
pointed at by end variables is not.
None of the members of a `PangoGlyphItemIter` should be modified manually.