seat: set cursor image only if no pointer cap previously
seat: set cursor image only if no pointer cap previously

This make seat_update_capabilities set cursor image only if
there was no pointer cap before update. This avoid resetting
cursor to left_ptr if an input device is removed.

1ace/sway,1ace/sway,ascent12/sway,ascent12/sway,SirCmpwn/sway,ascent12/sway,1ace/sway