search: wip: initial search matching

* match search buffer against scrollback content
* adjust view to ensure matched content is visible
* create selection on a successful match
* finalize selection when user presses enter (to "commit" the search)
* ctrl+r searches for the next match. Needs more work though.

terminal.h

@@ -336,6 +336,11 @@ struct terminal {
         wchar_t *buf;
         size_t len;
         size_t sz;
+
+        int original_view;
+        bool view_followed_offset;
+        struct coord match;
+        size_t match_len;
     } search;
 
     struct grid normal;