Include hstack_vaapi, vstack_vaapi and xstack_vaapi. They may accept
input streams with different sizes.  libva2 (VA-API 1.0+) is required.

Example:
$ ffmpeg -hwaccel vaapi -hwaccel_output_format vaapi -i input.h265 -filter_complex
"[0:v][0:v]hstack_vaapi" -c:v h264_vaapi out.h264

$ ffmpeg \
-hwaccel vaapi -hwaccel_output_format vaapi -i input.h264 \
-hwaccel vaapi -hwaccel_output_format vaapi -i input.h264 \
-hwaccel vaapi -hwaccel_output_format vaapi -i input.h264 \
-hwaccel vaapi -hwaccel_output_format vaapi -i input.h264 \
-filter_complex "[0:v][1:v][2:v][3:v]xstack_vaapi=inputs=4:fill=0x000000:layout=0_0_1920x1080|w0_0_1920x1080|0_h0_1920x1080|w0_h0_1920x1080" \
-c:v hevc_vaapi out.h265
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>