Read a DNG's NoiseProfile

The converter's measured noise for the body at that ISO, (S, O) per CFA
plane: the learned denoise's best source for a body with no table of its
own (denoise.md §3.3). Read from the header beside the colour tags, and
printed by rawinfo. Checked against tifffile on a 6D DNG at ISO 5000: all
six values agree.

core/dr-decode/src/lib.rs

@@ -544,6 +544,17 @@ pub(crate) fn parse_exif_offset(s: &str) -> Option<i32> {
     Some(sign * (h * 60 + m))
 }
 
+/// TRACES: FR-DEV-3g
+/// The DNG `NoiseProfile` of a file, if it carries one: `(S, O)` per CFA
+/// colour plane, variance `S·x + O` in black-to-white normalised units. See
+/// [`profile::read_noise_profile`]. Reads the header, not the image.
+pub fn noise_profile(bytes: &[u8]) -> Option<Vec<(f32, f32)>> {
+    use rawler::rawsource::RawSource;
+    let source = RawSource::new_from_slice(bytes);
+    let decoder = rawler::get_decoder(&source).ok()?;
+    profile::read_noise_profile(decoder.as_ref())
+}
+
 /// TRACES: FR-RAW-3 | FR-EXP-9
 /// Fully decode sensor data.
 ///