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.
A comment from the peanut gallery: I would definitely avoid refactoring 
code like this.  For one, it's an easy way for someone to inadvertently 
add bugs to the code.  And beyond that, by essentially making a fork of 
the original, it becomes unnecessarily hard to import newer versions of 
the code when the upstream version changes down the line.

Keep in mind that future maintainers may not have as good of an idea of 
what this code does, and where in the dcraw codebase it came from, as 
you do.

--xsdg