interactive: set grab parameters at cursor press

Add interactive_set_grab_context() which is called when the mouse button
is first pressed, before interactive_begin(). This fixes two small issues:

- The cursor origin position for interactive move/resize was slightly
  off (depending on mouse resolution), because it was set after the
  mouse had already moved slightly. Now it's exact.

- If app- or keybind-initiated maximize (etc.) happened after the button
  press but before the mouse was moved, then interactive_begin() would
  still start move/resize even though the view might now be far away
  from the cursor. Now interactive_cancel() works as expected, even if
  called before interactive_begin().

Also, make sure to call interactive_cancel() for un-maximize as well.

src/view.c

@@ -1433,14 +1433,11 @@ view_maximize(struct view *view, enum view_axis axis)
 	bool store_natural_geometry = !in_interactive_move(view);
 	view_set_shade(view, false);
 
-	if (axis != VIEW_AXIS_NONE) {
-		/*
-		 * Maximize via keybind or client request cancels
-		 * interactive move/resize since we can't move/resize
-		 * a maximized view.
-		 */
-		interactive_cancel(view);
-	}
+	/*
+	 * Maximize/unmaximize via keybind or client request cancels
+	 * interactive move/resize.
+	 */
+	interactive_cancel(view);
 
 	/*
 	 * Update natural geometry for any axis that wasn't already