input: allow mouse selections to start inside the margins

Before this, margins were special cased:

* The mouse cursor was always a pointer, and never an I-beam (thus
  signaling selections cannot be made).
* The internal mouse coords where set to -1 when the cursor was inside
  the margins, causing:
    - text selections from being made
    - mouse events being passed to mouse grabbing applications

In particular, even with a one-pixel margin, making selections was
unnecessarily hard in e.g. fullscreen mode, where you'd expect to be
able to throw the cursor into the corner of the screen and then start
a selection.

With this patch, the cursor is treated as if it was in the first/last
column/row, when inside the margin(s).

An unintended side-effect of this, initially, was that auto-scrolling
selections where way too easy to trigger, since part of its logic is
checking if the cursor is inside the margins.

That problem has been reduced by two things:

* auto-scrolling does not occur unless a selection has been
  started. That is, just holding down the mouse in the margins and
  moving up/down doesn't cause scrolling. You have to first select at
  least one cell in the visible viewport.
* A selection isn't fully started (i.e. a cell is actually selected)
  unless the cursor is inside the actual grid, and *not* in the
  margins.

What does the last point mean? We now allow a selection to be
_started_ when clicking in the margin. What this means internally is
we set a start coordinate for a selection, but *not* and end
coordinate. At this point, we don't have an actual selection. Nothing
is selected, and no cells are highlighted, graphically.

This happens when we set an end coordinate. Without the last bullet
point, that would happen as soon as the cursor was _moved_, even if
still inside the margins. Now, we require the cursor to leave the
margins and touch an actual cell before we set an end coordinate.

Closes #1702

terminal.c

@@ -3451,10 +3451,7 @@ term_xcursor_update_for_seat(struct terminal *term, struct seat *seat)
             shape = CURSOR_SHAPE_CUSTOM;
         }
 
-        else if (seat->mouse.col >= 0 &&
-                 seat->mouse.row >= 0 &&
-                 term_mouse_grabbed(term, seat))
-        {
+        else if (term_mouse_grabbed(term, seat)) {
             shape = CURSOR_SHAPE_TEXT;
         }