Do not rely on dt_get_thread_num in colorin module

dt_get_thread_num() can be the same value for multiple chunks under some circumstances. Using it for the start and end calculation will thus be unreliable, resulting in parts of the image containing just black pixels.

Fixes  #18127.

Author: mayjs

src/iop/colorin.c

@@ -936,7 +936,7 @@ static void process_cmatrix_fastpath(dt_iop_module_t *self,
   DT_OMP_FOR()
   for(size_t chunk = 0; chunk < nthreads; chunk++)
   {
-    size_t start = chunksize * dt_get_thread_num();
+    size_t start = chunksize * chunk;
     if(start >= npixels) continue;  // handle case when chunksize is < 4*nthreads and last thread has no work
     size_t end = MIN(start + chunksize, npixels);
     if(clipping)
@@ -1062,7 +1062,7 @@ static void process_cmatrix_proper(dt_iop_module_t *self,
   DT_OMP_FOR()
   for(size_t chunk = 0; chunk < nthreads; chunk++)
   {
-    size_t start = chunksize * dt_get_thread_num();
+    size_t start = chunksize * chunk;
     if(start >= npixels) continue;  // handle case when chunksize is < 4*nthreads and last thread has no work
     size_t end = MIN(start + chunksize, npixels);
     if(clipping)