ChafaTermDb: Go back to sixels as default graphics format for WezTerm

Unfortunately, its Kitty protocol implementation is not conformant,
which causes new placements to delete previous ones. This can be
worked around by specifying I=1, but then you'll also need to set q=1
to suppress responses. The latter is not implemented in WezTerm.

Reverts 444a367.

chafa/chafa-term-db.c

@@ -457,7 +457,7 @@ detect_capabilities (ChafaTermInfo *ti, gchar **envp)
 
     if (!g_ascii_strcasecmp (term_program, "WezTerm"))
     {
-        gfx_seqs = kitty_seqs;
+        gfx_seqs = sixel_seqs;
     }
 
     if (!g_ascii_strcasecmp (term_name, "contour"))