vaapi_encode_mjpeg: Use CBS to store parameters and write headers

Also adds greyscale, 4:2:2, 4:4:4 and RGB support.

configure

@@ -2974,7 +2974,7 @@ mjpeg_cuvid_decoder_deps="cuvid"
 mjpeg_qsv_encoder_deps="libmfx"
 mjpeg_qsv_encoder_select="qsvenc"
 mjpeg_vaapi_encoder_deps="VAEncPictureParameterBufferJPEG"
-mjpeg_vaapi_encoder_select="vaapi_encode jpegtables"
+mjpeg_vaapi_encoder_select="cbs_jpeg jpegtables vaapi_encode"
 mpeg1_cuvid_decoder_deps="cuvid"
 mpeg1_v4l2m2m_decoder_deps="v4l2_m2m mpeg1_v4l2_m2m"
 mpeg2_crystalhd_decoder_select="crystalhd"

doc/encoders.texi

@@ -2662,8 +2662,21 @@ Include access unit delimiters in the stream (not included by default).
 @end table
 
 @item mjpeg_vaapi
-Always encodes using the standard quantisation and huffman tables -
-@option{global_quality} scales the standard quantisation table (range 1-100).
+Only baseline DCT encoding is supported.  The encoder always uses the standard
+quantisation and huffman tables - @option{global_quality} scales the standard
+quantisation table (range 1-100).
+
+For YUV, 4:2:0, 4:2:2 and 4:4:4 subsampling modes are supported.  RGB is also
+supported, and will create an RGB JPEG.
+
+@table @option
+@item jfif
+Include JFIF header in each frame (not included by default).
+@item huffman
+Include standard huffman tables (on by default).  Turning this off will save
+a few hundred bytes in each output frame, but may lose compatibility with some
+JPEG decoders which don't fully handle MJPEG.
+@end table
 
 @item mpeg2_vaapi
 @option{profile} and @option{level} set the value of @emph{profile_and_level_indication}.