Allow creation of default Baseline JPEG image: JPGFLAG_BASELINE

[malaterre]

jpeg command line tool does not allow creation of default baseline JPEG, it always default to sequential extended:

• https://github.com/thorfdbg/libjpeg/blob/master/cmd/encodec.cpp#L345

int frametype = JPGFLAG_SEQUENTIAL;

There is no way then to reset back frametype to JPGFLAG_BASELINE.

Typical setup (create an 8x8 RGB black image):

$ convert -depth 8 -size 8x8 xc:black black.ppm
$ jpeg -h -s 1x1,2x2,2x2 -q 100 black.ppm black_jpeg.jpg

compare to the ijg/turbo-jpeg:

$ cjpeg -optimize -quality 100 -outfile black_cjpeg.jpg black.ppm

libjpeg it creating:

• SOF1Huffman Extended Sequential DCT

while ijg/turbo-jpeg creates:

• SOF0Huffman Baseline DCT

(as a side note it would also be nice to be able to remove the APPBmarker completely).

[thorfdbg]

Added a "-bl" option to enforce encoding in the baseline mode. Please test and re-open if this does not work as expected.