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.
Thank you for the sharp eye on this! Checking if `max==0.0f` could be OK as it's really checking if 0.0f has been assigned to `max` somewhere else in the loop (as max is overloaded, serving both as a max vlaue and as a flag that it needs to be calculated)? But this whole section is awkward, due to my less-than-ideal conversion of dcraw's code to handle floats rather than ints. I'll work up an overall clean-up/clarification instead...