url-mode: snapshot screen state when entering URL mode

Previously, we automatically exited URL mode whenever we received data
on the PTY. This was done since we don’t know _what_ has changed on
the screen, and we don’t want to display misleading jump labels.

However, this becomes a problem in curses-like applications that
periodically updates part of the screen. For example, a statusbar with
a clock.

This patch changes this behavior; instead of cancelling URL mode when
receiving PTY data, we snapshot the grid when entering URL mode.

When *rendering*, we use the snapshot:ed grid, while PTY updates
modify the “real” grid.

Snapshot:ing the grid means taking a full/deep copy of the current
grid, including sixel images etc.

Finally, it isn’t necessary to “damage” the entire view
when *entering* URL mode, since we’re at that point the renderer is in
sync with the grid. But we *do* need to damage the entire view when
exiting URL mode, since the grid changes on the “real” grid hasn’t
been tracked by the renderer.

terminal.h

@@ -547,9 +547,11 @@ struct terminal {
         unsigned max_height;    /* Maximum image height, in pixels */
     } sixel;
 
+    /* TODO: wrap in a struct */
     url_list_t urls;
     wchar_t url_keys[5];
     bool urls_show_uri_on_jump_label;
+    struct grid *url_grid_snapshot;
 
 #if defined(FOOT_IME_ENABLED) && FOOT_IME_ENABLED
     struct {