[transfer-syntax-registry] Set Photometric Interpretation to YBR_FULL_422 when encoding JPEG Baseline

[userepo]

Fixes #829.

• When encoding a 3-sample-per-pixel object to JPEG Baseline, the frame is handed to jpeg-encoder as ColorType::Rgb, which converts the samples to YCbCr and writes a JFIF stream. The attribute changes returned by encode_frame nevertheless asked for (0028,0004) Photometric Interpretation to be RGB. Request YBR_FULL_422 instead, since that is the color space of the encoded data.
• RGB is not an invalid value here — Table 8.2.1-1 in PS3.5 Section 8.2.1 lists both YBR_FULL_422 and RGB for the 3-sample row of JPEG Baseline. What the same section does require is that the Data Elements describing the Pixel Data encoding be consistent with the characteristics of the compressed data stream, and a RGB tag over a YCbCr codestream is not.