Some more GUI fixes

- Try to keep background grids crisp in custom widgets.
- Put a subdued dt logo into the empty widget of zone system.
update to LibRaw 0.16.0, load x-trans images

- Use LibRaw 0.16.0 to load x-trans images
- LibRaw >= 1.15.0 no longer allows document_mode (which returned
  grayscale uninterpolated data), hence now receive multi-channel
  data then copy it to dt single-channel mosaic format
- Cull unneeded libraw parameters which pertain to output.
- Libraw scales colors and subtracts black point, so no need to do this
  in dt_imageio_open_raw()
- For x-trans images, on image load orient/shift the mosaic pattern
  depending on orientation & usable image area.
- No longer blacklist x-trans raw files
fast downsampling to demosaic x-trans images
linear demosaic for x-trans files
highlights module for x-trans
temperature iop for x-trans
hotpixels for xtrans
raw denoise for x-trans

Use per-channel nearest-neighbor interpolation rather than downsizing
the channel.
invert module for x-trans

Also flip roi_in x/y for some bayer cases and don't divide by
0xffff in bayer float case (typos?).
disable ca correct module for x-trans
white balance presets for x100s
add measured fujifilm x100s basecurve
selectively update coeffs for x-trans sensors

All coefficients from dcraw 9.20 except for Fujifilm X-T1 camera matrix
from dcraw 9.21. Adding by hand as auto-generation via update_dcraw.sh
seems to produce unusable file (c.f. capitalization problems).
DMC-GH1 aspect ratio for comparison testing
disable green-matching in demosaic for x-trans
more demosaic tuning for x-trans

- don't use Bayer-only edge-aware median for < 50% scaled exports
- hide Bayer-only options in GUI
for x-trans don't downsample if scale > 33.3%

For Bayer this is 50%. For x-trans, the fast down-sample works off of
3x3 sensor matrix (1/4 of whole sensor), rather than 2x2 as with
Bayer.
first pass at Markesteijn demosaic for x-trans

Not ready for use! For now this is pretty much the dcraw code wrapped
to work w/in demosaic module. The demosaic pipeline data is converted
to dcraw-friendly format for processing, then converted back
post-processing. The dcraw code is 16-bit integer, rather than
floating point. Provisional and slow, but still noticeably better than
bilinear. Many other pieces need clean-up (c.f. the RGB to LAB
conversion).
retrieve camera matrix for Markesteijn demosaic

Up until now the matrix was a hardcoded hack. Markesteijn does some
work in Lab, and in the dcraw version does this mindful of the camera
matrix, so do the same, falling back to using linear RGB if can't find
the matrix. Linear RGB looks fine, and this is lots of extra code from
dcraw, so should probably either always use linear RGB or use extant
dt code to handle this.
differentiate bayer/xtrans demosaic methods

Previously the same method ids meant different things depending on if
the file had a Bayer or xtrans matrix. Now use a bit-mask to
differentiate the xtrans methods.