dcraw: add Panasonic DC-GH6

[sgotti]

dcraw: add Panasonic DC-GH6 to adobe_coeffs

Depends on v8 decoder provided by #7018

Tested images

• Tested normal images.
• Tested in-camera pixelshift high resolution images

[Lawrence37]

The decoder already has the white level for this camera.
white levels: R:65535 G1:65535 B:65535 G2:65535 (provided by dcraw)

[sgotti]

Right.

If useful this isn't directly provided by any metadata but calculated from tiff_bps (16 in this case):

RawTherapee/rtengine/dcraw.cc

Line 10720 in 5a25619

maximum = ((uint64_t)1 << tiff_bps) - 1; // use uint64_t to avoid overflow if tiff_bps == 32

[Lawrence37]

Similar to the edit to my comment in the other Panasonic camconst PR, Libraw has the color matrix and white level. We should consider copying the new Libraw constants for all cameras into dcraw.cc.
https://github.com/LibRaw/LibRaw/blob/158e635e5e80a95dfa402b0a79a7167922c1d4ec/src/tables/colordata.cpp#L1387-L1388

[sgotti]

@Lawrence37 Updated the PR to update the dcraw adobe_coeffs table instead of a camconst entry.
Note that the black level is set to 0, while in libraw is set to -2048 because the logic has changed a lot in libraw. If we set it to 2048 here we'll get a black level of 4096 since it's summed to the already provided single channel blacks levels (cblack) with value 2048 read from metadata.