Give the denoise work's test images a baseline exposure

The learned-denoise branch merged while this one was open, and two of
its test fixtures build a RawImage without the baseline_exposure field
this branch added; zero is the no-op value.

core/dr-gpu/tests/grain.rs

@@ -26,6 +26,7 @@ fn like(ctx: &GpuContext) -> DemosaicedImage {
         samples_per_pixel: 1,
         profile: None,
         profile_tables: None,
+        baseline_exposure: 0.0,
         make: String::new(),
         model: String::new(),
         crop: CropRect {

ui/dr-ui/src/develop/denoise.rs

@@ -329,6 +329,7 @@ mod tests {
             samples_per_pixel: 1,
             profile: None,
             profile_tables: None,
+            baseline_exposure: 0.0,
             make: String::new(),
             model: String::new(),
             crop: dr_decode::CropRect {