render: sixel: regression: need to take current offset into account when early-quitting sixel rendering

The sixel images are sorted, that's true. But in order for our row
numer comparisons to actually work, we need to rebase all numbers
against the current scrollback offset (or, the scrollback *end*, to be
precise).

render.c

@@ -795,20 +795,30 @@ render_sixel(struct terminal *term, pixman_image_t *pix,
 static void
 render_sixel_images(struct terminal *term, pixman_image_t *pix)
 {
-    const int view_start = term->grid->view;
-    const int view_end = view_start + term->rows;
+    if (likely(tll_length(term->grid->sixel_images)) == 0)
+        return;
 
-    LOG_DBG("SIXELS: %zu images, view=%d-%d",
-            tll_length(term->grid->sixel_images), view_start, view_end);
+    const int scrollback_end
+        = (term->grid->offset + term->rows) & (term->grid->num_rows - 1);
+
+    const int view_start
+        = (term->grid->view
+           - scrollback_end
+           + term->grid->num_rows) & (term->grid->num_rows - 1);
+
+    const int view_end = view_start + term->rows - 1;
+
+    //LOG_DBG("SIXELS: %zu images, view=%d-%d",
+    //        tll_length(term->grid->sixel_images), view_start, view_end);
 
     tll_foreach(term->grid->sixel_images, it) {
         const struct sixel *six = &it->item;
-        const int start = six->pos.row;
+        const int start
+            = (six->pos.row
+               - scrollback_end
+               + term->grid->num_rows) & (term->grid->num_rows - 1);
         const int end = start + six->rows - 1;
 
-        /* Sixels aren't allowed to cross the wrap-around */
-        assert(end < term->grid->num_rows);
-
         //LOG_DBG("  sixel: %d-%d", start, end);
         if (start > view_end) {
             /* Sixel starts after view ends, no need to try to render it */