selection: allow selections to pivot around a range instead of a point

Extend selection pivoting to allow selections to pivot around a
range.

Use this in word- and row-based selections to pivot around the initial
word/row that was selected.

This mimics the behavior of at least urxvt and xterm.

terminal.h

@@ -367,6 +367,11 @@ struct terminal {
         bool ongoing;
         bool spaces_only; /* SELECTION_SEMANTIC_WORD */
 
+        struct {
+            struct coord start;
+            struct coord end;
+        } pivot;
+
         struct {
             int fd;
             int col;