Upload the learned demosaic's result, and blend grain back into it

DemosaicedImage::from_rgb_f32 takes the network's linear camera RGB and
stands it beside the classical source of the same photograph: the matrix,
profile tables and as-shot balance are that source's, the id is new, so
nothing downstream can tell which demosaic ran and every cache keyed on the
source sees a new one.

GrainBlend is the denoise's live control. It returns only the brightness of
the noise the network removed, taken after the as-shot balance and handed
back divided by it, so the grain is neutral in the finished picture; colour
speckle and demosaic false colour stay out. It writes a new source rather
than adding a term to the adjust shader: the blend depends on two images and
one number, a 20 MP pass is milliseconds, and a fresh source id is all the
adjust pass's caches need. The test reads it back: at 0 the network's
result, at 1 the same white-balanced step in every channel.

core/dr-gpu/src/lib.rs

@@ -26,6 +26,7 @@ mod demosaic;
 mod detail;
 mod error;
 mod focus;
+mod grain;
 mod histogram;
 mod mask;
 mod merge;
@@ -41,6 +42,7 @@ pub use demosaic::{DemosaicedImage, Demosaicer};
 pub use detail::INTERMEDIATE_FORMAT as DETAIL_INTERMEDIATE_FORMAT;
 pub use error::GpuError;
 pub use focus::{FocusPeakPass, FocusPeaking, PeakColour, PeakSensitivity};
+pub use grain::GrainBlend;
 pub use merge::{Band, MergeFrame, MergeOutput, MergePass};
 // Renamed on the way out: `BINS` says enough inside `histogram`, and nothing
 // at all at a crate root shared with demosaic and segmentation.