please add new rw2 preview image tag support

[wbzqe]

please add rw2 tag "JpgFromRaw2" support to get bigger preview image for some rw2 image

exiv2 process rw2 tag PreviewImage 0x002e

according to exiftool, another tagid of preview image for rw2 is 0x0127 PanasonicRaw

I tried it myself and was able to successfully read it

1. add tag id to src/panasonicmn_int.cpp

...
{0x0118, "RawDataOffset", N_("Raw Data Offset"), N_("Raw data offset"), IfdId::panaRawId, SectionId::panaRaw,
     unsignedLong, -1, printValue},
    {0x0127, "JpgFromRaw2", N_("JpgFromRaw2"), N_("JpgFromRaw2"), IfdId::panaRawId, SectionId::panaRaw, undefined,
     -1, printValue},
    {0x8769, "ExifTag", N_("Exif IFD Pointer"), N_("A pointer to the Exif IFD"), IfdId::panaRawId, SectionId::panaRaw,
     unsignedLong, -1, printValue},
...

2. add tag info to src/preview.cpp

const Loader::LoaderList Loader::loaderList_[] = {
...
{"image/x-panasonic-rw2", createLoaderExifDataJpeg, 12},
...
}
....
const LoaderExifDataJpeg::Param LoaderExifDataJpeg::param_[] = {
....
    {"Exif.PanasonicRaw.JpgFromRaw2", nullptr},                                                        // 12
};

Thank you very much for your help.