sixel: special case parsing of images with an aspect ratio of 1:1

Images with an aspect ratio of 1:1 are by far the most common (though
not the default).

It makes a lot of sense, performance wise, to special case
them.

Specifically, the sixel_add() function benefits greatly from this, as
it is the inner most, most heavily executed function when parsing a
sixel image.

sixel_add_many() also benefits, since allows us to drop a
multiplication. Since sixel_add_many() always called first (no other
call sites call sixel_add() directly), this has a noticeable effect on
performance.

Another thing that helps (though not as much), and not specifically
with AR 1:1 images, is special casing DECGRI a bit.

Up until now, it simply updated the current sixel parameter value. The
problem is that the default parameter value is 0. But, a value of 0
should be treated as 1. By adding a special ‘repeat_count’ variable to
the sixel struct, we can initialize it to ‘1’ when we see DECGRI, and
then simply overwrite it as the parameter value gets updated. This
allows us to drop an if..else when emitting the sixel.

dcs.c

@@ -427,8 +427,7 @@ dcs_hook(struct terminal *term, uint8_t final)
             int p2 = vt_param_get(term, 1,0);
             int p3 = vt_param_get(term, 2, 0);
 
-            sixel_init(term, p1, p2, p3);
-            term->vt.dcs.put_handler = &sixel_put;
+            term->vt.dcs.put_handler = sixel_init(term, p1, p2, p3);
             term->vt.dcs.unhook_handler = &sixel_unhook;
             break;
         }