fix(IBA): demosaic - fix roi channels (AcademySoftwareFoundation#4602)

Fixes a bug in demosaicing where the code ignores the ROI channels.
The old behaviour was to always decode into the first 3 channels.

Signed-off-by: Anton Dukhovnikov <[email protected]>
Signed-off-by: Scott Wilson <[email protected]>

Author: antond-weta

src/libOpenImageIO/imagebufalgo_demosaic.cpp

@@ -1019,12 +1019,11 @@ demosaic(ImageBuf& dst, const ImageBuf& src, KWArgs options, ROI roi,
 
     ROI dst_roi = roi;
     if (!dst_roi.defined()) {
-        dst_roi = src.roi();
+        dst_roi         = src.roi();
+        dst_roi.chbegin = 0;
+        dst_roi.chend   = 3;
     }
 
-    dst_roi.chbegin = 0;
-    dst_roi.chend   = 2;
-
     ImageSpec dst_spec = src.spec();
     dst_spec.nchannels = 3;
     dst_spec.default_channel_names();
@@ -1071,7 +1070,7 @@ demosaic(ImageBuf& dst, const ImageBuf& src, KWArgs options, ROI roi,
         dst.errorfmt("ImageBufAlgo::demosaic() invalid pattern");
     }
 
-    return true;
+    return ok;
 }
 
 ImageBuf