input: margins are not selectable

* Fix col/row calculation in pointer-enter event; we did not take the
  margins into account.
* seat->mouse.col,row are now set to -1 if the cursor is inside the
  margins. That is, col/row are only ever valid when the mouse is
  actually over the grid, and not in the margins.
* Use regular 'left-ptr' mouse cursor when mouse is inside the
  margins, to not make the user think he/she can start a selection.

Besides making things clearer, this also fixes a crash that occurred
if you started a selection in e.g. the right margin.

selection.c

@@ -25,9 +25,10 @@ bool
 selection_enabled(const struct terminal *term, struct seat *seat)
 {
     return
-        term->mouse_tracking == MOUSE_NONE ||
-        term_mouse_grabbed(term, seat) ||
-        term->is_searching;
+        seat->mouse.col >= 0 && seat->mouse.row >= 0 &&
+        (term->mouse_tracking == MOUSE_NONE ||
+         term_mouse_grabbed(term, seat) ||
+         term->is_searching);
 }
 
 bool