CR3 support

This is basically from darktable-org#271

Author: axxel

src/librawspeed/decoders/CMakeLists.txt

@@ -7,6 +7,8 @@ FILE(GLOB DECODER_SOURCES
   "ArwDecoder.h"
   "Cr2Decoder.cpp"
   "Cr2Decoder.h"
+  "Cr3Decoder.cpp"
+  "Cr3Decoder.h"
   "CrwDecoder.cpp"
   "CrwDecoder.h"
   "DcrDecoder.cpp"