New public API:
cairo_text_cluster_t
cairo_has_show_text_glyphs()
cairo_show_text_glyphs()
Add accompanying gstate and surface functions, and surface backend methods.
No backends implement them just yet.
It was reported by Liu Yubao that cairo_status_t may be chosen to be
an int8_t by the compiler, in that case cairo_int_status_t values
assigned to cairo_status_t would overflow. Fix this by allocating
the values in int8_t range, and add compile-time sanity checks.
