support X-Trans sensor raw files

X-Trans specific changes:

- use RawSpeed to read RAF files
- downsampling for fast display
- interpolation: bilinear, VNG, and modified AHD (aka Markesteijn), all adapted from dcraw; these operate on floats and use OpenMP
- highlights, hotpixels, invert, rawdenoise, temperature handle X-Trans
- disable cacorrect for X-Trans
- white balance presets and basecurve for X100S
- cameras.xml and adobe_coeff.c entries X-Series cameras
- documentation updates
- dngmeta.sh handles X-Trans

None of these changes should alter performance for users of Bayer sensors. The changes have been tested on Bayer and X-Trans matrix raws as well LDR files.

The code in this pull request doesn't:

- handle "dynamic range" feature on X-Series cameras
- implement SSE/OpenCL optimizations
- implement cacorrect for X-Trans sensors
- include white balance data and basecurves for X-Series cameras besides X100S

There is generally one commit for each major change. For the VNG/Markesteijn demosaic code, one commit imports/wraps code from dcraw, then another modifies this code to use floats/OpenMP and otherwise make it more specific to darktable.

This pull request closes #9951 and fixes #9388.

If it becomes necessary to update to the latest LibRaw, see separate branch libraw-0.16 (https://github.com/dtorop/darktable/tree/libraw-0.16). This pull request for the "xtrans2" branch keeps the extant LibRaw 0.14.7 (with the addition of a small patch to not break the temperature iop for the X-Pro1) so as not to add another LibRaw dependency.
Am still skeptical about writing `shared (out)` which seems to share the pointer but not the underlying data. Not to mention that as `out` is declared `float *const out`, OpenMP doesn't allow sharing of it anyhow without dropping the `const`.

That said, declaring `out` as `float *out` and `shared(out)` is in keeping with the rest of the dt code. If this is questionable, better that I take it up overall sometime later. There seems to be no performance difference with shared() or without. As I'm new to OpenMP, I'm happy to defer here, rather than being (except in this note) pedantic.

(Does sharing `out` create a critical section around writes to the memory to which `out` points? Not that that's necessary in this case, as each thread writes to its own section of memory.)