fix(raw): fix LibRaw flip to Exif orientation conversion (#3847)

When reading a raw image while having a `user_flip` set to 1, we set the
orientation Exif tag to LibRaw's flip value (so that LibRaw doesn't
rotate and/or mirror the buffer but the image still gets displayed
correctly).

However LibRaw's convention for flip values differ from Exif orientation
values: https://www.libraw.org/comment/5176#comment-5176.

In this PR we fix this issue by converting LibRaw flip values to Exif
orientation values. This conversion is based on:
- https://www.libraw.org/comment/5178#comment-5178 (conversion proposed
by LibRaw's maintainer)
-
https://github.com/LibRaw/LibRaw/blob/6fffd414bfda63dfef2276ae07f7ca36660b8888/src/write/file_write.cpp#L22
(LibRaw's function for applying the flip).

Tested on:
- CR2 images with flip values 1, 5 and 6
- ARW images with flip values 1 and 5
(unfortunately I could not find any image covering other flip values).

Since the conversion is `[1, 2, 3, 4, 5, 6, 7, 8] -> [1, 2, 4, 3, 5, 8,
6, 7]`, the only test that came out with different results before and
after the change is the one with flip value 6:
in that case, the image orientation got corrected by the fix.

src/raw.imageio/rawinput.cpp

@@ -433,8 +433,15 @@ RawInput::open_raw(bool unpack, const std::string& name,
         m_unpacked = true;
     }
 
-    //Store flip before it is potentially overwritten
-    int original_flip = m_processor->imgdata.sizes.flip;
+    // Store flip before it is potentially overwritten
+    // LibRaw's convention for flip values differs from Exif orientation tags
+    // so we need to convert it
+    int original_flip                 = 1;
+    int libraw_flip                   = m_processor->imgdata.sizes.flip;
+    static int libraw_to_exif_flip[8] = { 1, 2, 4, 3, 5, 8, 6, 7 };
+    if (libraw_flip >= 1 && libraw_flip <= 8) {
+        original_flip = libraw_to_exif_flip[libraw_flip - 1];
+    }
     m_processor->adjust_sizes_info_only();
 
     // Process image at half size if "raw:half_size" is not 0