compositor: weston_view_assign_output is changed to the function pointer.

Allow us to select behavior of weston_view_assign_out. E.g. to support
multi display in non-stride mode.

Signed-off-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>
compositor: add screen mode, non-stride

For the time being, if there are two screens, it is regards as like
extended display of PC. However there is a use case of IVI to handle
two screens separately.

Signed-off-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>
gl-renderer: repaint_views is changed to the function pointer

To support non-stride mode of multi screen.

Signed-off-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>
weston.ini.in: Append a stride-screen option to weston.ini.in

Default value is true. It will not affect the exist behavior.

Signed-off-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>