lavc/vaapi_encode: add tile slice encoding support

lavc/vaapi_encode: add tile slice encoding support

Add functions to initialize tile slice structure and make tile slice:
 - vaapi_encode_init_tile_slice_structure
 - vaapi_encode_make_tile_slice

Tile slice is not allowed to cross the boundary of a tile due to
the constraints of media-driver. Currently adding support for one
slice per tile.

N x N tile encoding is supposed to be supported with the the
capability of ARBITRARY_MACROBLOCKS slice structures.

N X 1 tile encoding should also work in ARBITRARY_ROWS slice
structure.

Signed-off-by: Linjie Fu <linjie.justin.fu@gmail.com>