lavfi/vf_scale_vaapi: add scaling mode setting support.

before this change, scale_vaapi hard coding the scaling mode, add a
new option "mode" to setting the scaling mode, it can be use to change
scaling algorithm for performance/quality trade off.

Note: VA_FILTER_SCALING_NL_ANAMORPHIC(nolinear ananorphic scaling
algorithm) doesn't support by i965/iHD driver now, add this just for
completeness.

Reviewed-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: Jun Zhao <mypopydev@gmail.com>
