trunk 1.8.0 regression with internal libtiff and JPEG compression

[Even Rouault]

gdal_translate ../autotest/gcore/data/rgbsmall.tif  tif_jpeg.tif -co COMPRESS=JPEG

produces a dataset that cannot be read :

$ gdalinfo -checksum tif_jpeg.tif
Driver: GTiff/GeoTIFF
Files: tif_jpeg.tif
Size is 50, 50
Coordinate System is:
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563,
            AUTHORITY["EPSG","7030"]],
        AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0],
    UNIT["degree",0.0174532925199433],
    AUTHORITY["EPSG","4326"]]
Origin = (-44.840319999999998,-22.932583999999999)
Pixel Size = (0.003432000000000,-0.003432000000000)
Metadata:
  AREA_OR_POINT=Area
Image Structure Metadata:
  COMPRESSION=JPEG
  INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left  ( -44.8403200, -22.9325840) ( 44d50'25.15"W, 22d55'57.30"S)
Lower Left  ( -44.8403200, -23.1041840) ( 44d50'25.15"W, 23d 6'15.06"S)
Upper Right ( -44.6687200, -22.9325840) ( 44d40' 7.39"W, 22d55'57.30"S)
Lower Right ( -44.6687200, -23.1041840) ( 44d40' 7.39"W, 23d 6'15.06"S)
Center      ( -44.7545200, -23.0183840) ( 44d45'16.27"W, 23d 1' 6.18"S)
Band 1 Block=50x50 Type=Byte, ColorInterp=Red
Warning 1: JPEGPreDecode:Improper JPEG sampling factors 2,2
Apparently should be 1,1.
ERROR 1: JPEGPreDecode:Cannot honour JPEG sampling factors that exceed those specified.
ERROR 1: TIFFReadEncodedStrip() failed.
ERROR 1: IReadBlock failed at X offset 0, Y offset 0
ERROR 1: GetBlockRef failed at X block offset 0, Y block offset 0
ERROR 3: Checksum value couldn't be computed due to I/O read error.

  Checksum=0
Band 2 Block=50x50 Type=Byte, ColorInterp=Green
  Checksum=0
Band 3 Block=50x50 Type=Byte, ColorInterp=Blue
  Checksum=0

Works fine with 1.7 branch and its internal libtiff.

The issue occurs when writing the dataset (a dataset produced with 1.7 can be read by trunk, but not the reverse)