wayland: Always request transient commit when mapping subsurface

wayland: Always request transient commit when mapping subsurface

A subsurface positioning operation only takes effect when the parent
surfaces state is applied. If a subsurface is mapped and positioned, but
the parent surface state is not immediately committed, the relative
position of the subsurface is undefined and may be placed incorrectly.

To avoid this undefined state, always request that the parent surface
should be committed after mapping a subsurface so that the position
operation will take effect.

https://bugzilla.gnome.org/show_bug.cgi?id=751098