compositor: Set priv->current_drag before calling meta_window_drag_begin()

priv->current_drag is the property that meta_compositor_get_current_window_drag() and therefore also meta_display_is_grabbed() bases its check on. We use meta_display_is_grabbed() to select which cursor to use (window cursor vs root cursor) when updating the cursor in MetaCursorTracker.

Since meta_window_drag_begin() sets a new cursor, and therefore triggers the cursor tracker to update the current visible cursor, we should set priv->current_drag before the call to meta_window_drag_begin(). This makes sure the cursor tracker sees that there's a window drag and changes the cursor right away when the window drag begins (instead of doing so on subsequent pointer events).

Fixes: 525ed116 ("wayland/pointer: Unset current surface during window drags")