jpegdec: Support libjpeg-turbo colorspace conversion

The libjpeg-turbo has a built-in support for performing colorspace conversion. The performance of this conversion is much better than doing the same separately using videoconvert. Implement support for this conversion to RGBx/xRGB/BGRx/xBGR formats. Other formats can be easily added later.

• The decoding of various pixel formats can be tested and compared to non-libjpeg-turbo decoding as follows:

  for gfmt in {RGB,BGR}{,x} x{RGB,BGR} ; do
      echo "$gfmt"
      gst-launch-1.0 -q \
          videotestsrc pattern=colors ! \
          video/x-raw,format=${gfmt} ! \
          fakesink dump=true | \
	  head -n 200 | tail -n 1
      gst-launch-1.0 -q --gst-plugin-path=build/ext/jpeg/ \
          videotestsrc pattern=colors ! \
          video/x-raw,format=${gfmt} ! \
	  jpegenc ! \
	  jpegdec ! \
	  video/x-raw,format=${gfmt} ! \
	  fakesink dump=true | \
	  head -n 200 | tail -n 1
      done

Result looks as follows, i.e. comparable:

  RGB
  00000c70 (0x7f7736fbdd10): 05 33 19 05 33 26 05 33 33 05 33 40 05 33 4c 05  .3..3&.33.3@.3L.
  00000c70 (0x7f389e8f7d10): 05 32 17 04 32 22 04 32 31 04 32 3e 04 32 4a 04  .2..2".21.2>.2J.
  RGBx
  00000c70 (0x7f79efd0ad10): cc 07 22 ff d9 07 22 ff e6 07 22 ff f3 07 22 ff  .."..."..."...".
  00000c70 (0x7fb6989f3d10): cd 06 22 00 d9 06 22 00 e6 06 22 00 f4 06 22 00  .."..."..."...".
  BGR
  00000c70 (0x7fa0a6c42d10): 05 0c 33 05 19 33 05 26 33 05 33 33 05 40 33 05  ..3..3.&3.33.@3.
  00000c70 (0x7fc74165fd10): 05 08 32 04 17 32 04 22 32 04 31 32 04 3e 32 04  ..2..2."2.12.>2.
  BGRx
  00000c70 (0x7fbf399f1d10): 22 07 cc ff 22 07 d9 ff 22 07 e6 ff 22 07 f3 ff  "..."..."..."...
  00000c70 (0x7f50e3d1cd10): 22 06 cd 00 22 06 d9 00 22 06 e6 00 22 06 f4 00  "..."..."..."...
  xRGB
  00000c70 (0x7f0b950a2d10): ff cc 07 22 ff d9 07 22 ff e6 07 22 ff f3 07 22  ..."..."..."..."
  00000c70 (0x7f4416b8dd10): 00 cd 06 22 00 d9 06 22 00 e6 06 22 00 f4 06 22  ..."..."..."..."
  xBGR
  00000c70 (0x7f237d74dd10): ff 22 07 cc ff 22 07 d9 ff 22 07 e6 ff 22 07 f3  ."..."..."..."..
  00000c70 (0x7f095547dd10): 00 22 06 cd 00 22 06 d9 00 22 06 e6 00 22 06 f4  ."..."..."..."..
                             ^^          ^^          ^^          ^^

Notice how the alpha channel is set to arbitrary value in case of the libjpeg-turbo decoding into RGBx/BGRx/xRGB/xBGR pixel formats. This is documented in libjpeg-turbo README.md as follows: " When using the RGBX, BGRX, XBGR, and XRGB colorspaces during decompression, the X byte is undefined, and in order to ensure the best performance, libjpeg-turbo can set that byte to whatever value it wishes. "

• The interlaced num_fields=2 mjpeg stream can be generated and tested as follows (this does require mjpegtools):

  $ gst-launch-1.0 videotestsrc num-buffers=10 ! jpegenc ! multifilesink location=in%04d.jpg
  $ jpeg2yuv -f 25 -I t -L 0 -j in%04d.jpg | yuv2lav -f avi -o result.avi
  ...
  $ gst-launch-1.0 --gst-plugin-path=build/ext/jpeg/ filesrc location=result.avi ! \
       avidemux ! jpegdec ! video/x-raw,format=RGBx ! videoconvert ! autovideosink