cjpeg: Only support 8-bit precision w/ GIF input

Creating 12-bit-per-sample JPEG images from GIF input images was a
useful testing feature when the data precision was a compile-time
setting.  However, now that the data precision is a runtime setting,
it doesn't make sense for cjpeg to allow data precisions other than
8-bit with GIF input images.  GIF images are limited to 256 colors from
a palette of 8-bit-per-component RGB values, so they cannot take
advantage of the additional gamut afforded by higher data precisions.

ChangeLog.md

@@ -13,6 +13,11 @@ been moved into a new **src/** subdirectory.
 performance of compressing/decompressing a grayscale JPEG image from/to a
 packed-pixel grayscale image.
 
+3. cjpeg no longer allows GIF input files to be converted into
+12-bit-per-sample JPEG files.  That was never a useful feature, since GIF
+images have at most 256 colors referenced from a palette of 8-bit-per-component
+RGB values.
+
 
 3.0.4
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