Update internal Libraw to 0.18.0 beta (July 2016).

Move Libraw samples compilation rules to core/tests/rawengine. Theses samples
files are now only generated if test code are compiled through right CMake option.

Use DNG SDK source code to compile Libraw in goal to:
   - Decode DNG analog color balances
   - Decode DNG channel black/white level
   - 8-bit encoded DNG files
   - Deflate compressed DNG support with zlib

New features:
   - Support for Black and White Jpeg previews
   - Better intepolation of low-sensitivity pixels on Sigma Quattro files

New RAW cameras supported:
   - BlackMagic Micro Cinema Camera, URSA Mini
   - Canon 80D, 1300D, 1DX MkII
   - Fujifilm X-Pro2 (uncompressed RAW only), X70, X-E2S
   - HTC One A9 and M9
   - Leica M (Typ 262) and X-U (Typ 113)
   - Nikon D5
   - Olympus Pen F, SH-3
   - Panasonic DMC-GX80/85, DMC-TZ80/81/85/ZS60, DMC-TZ100/101/ZS100
   - PhaseOne IQ150, IQ180 IR, IQ3 (all models)
   - Samsung Galaxy S7 (SM-G935F)
   - Sony ILCA-68, ILCE-6300, RX1R II, RX10 III
   - YUNEEC CGO4

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