Add JPEG lossless data set 2

- covers 12-bit and 10-bit precision lossless JPEG decoding

Author: Enet4

tests/reftest/images/lossless/2/MR4.jpg

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+# JPEG Lossless data set 2
+
+The JPEG losssless files in this data set
+were collected from the DICOM-WG04 compilation of public DICOM files in
+<ftp://medical.nema.org/medical/dicom/DataSets/WG04> (revision 2004/08/26),
+extracting the JPEG data from the files' pixel data.
+
+The ground truth was collected from the respective reference (uncompressed) files
+and converted to 16-bit PNG files.
+
+## Description
+
+- **MR4**: 512x512, 12 bit precision, non-hierarchical, selection value 1
+  - JPEG extracted from DICOM file `JPLL/MR4_JPLL`
+  - Ground truth converted from DICOM file `REF/MR4_UNC`
+- **XA1**: 1024x1024, 10 bit precision, non-hierarchical, selection value 1
+  - JPEG extracted from DICOM file `JPLL/XA1_JPLL`
+  - Ground truth converted from DICOM file `REF/XA1_UNC`