A linear DNG in and out: the writer, and a three-sample RawImage

dr-export gains write_linear_dng — LinearRaw, DNG 1.4, u16 samples at
the sensor's scale, the body's matrices with their illuminants, the
as-shot neutral, the EXIF block an export writes — streamed strip by
strip through a closure so the composite is never held (FR-MRG-11). The
tiff crate's directory is a map, so PhotometricInterpretation is written
over what new_image set, which is the trick the S15.1 spike thought it
had to hand-roll around. The test reads the file back through rawler.

dr-decode's RawImage carries samples_per_pixel (a linear DNG is 3), the
body's profile with its calibrations mapped back to EXIF illuminant
codes, and the cleaned make and model. The GPU uploads a three-sample
image as it is, normalised by black and white like a photosite, through
a full f16 conversion — subnormals kept, because a 14-bit LSB sits at
f16's smallest normal and rounding it to zero would crush exactly the
shadows the file was written to keep.

core/dr-gpu/tests/film_sim.rs

@@ -46,6 +46,10 @@ fn flat_raw(level: u16) -> RawImage {
         // leaving a curve here would test the suppression rather than the
         // film. `dr-pipeline` asserts the suppression on the generated source.
         base_curve: BaseCurve::IDENTITY,
+        samples_per_pixel: 1,
+        profile: None,
+        make: String::new(),
+        model: String::new(),
         crop: CropRect {
             x: 0,
             y: 0,